How Long Do Kidney Stones Stay in the Kidney?

Kidney stones can remain lodged in the kidney for months, years, or even decades without causing any symptoms. Unlike the dramatic, painful event most people associate with kidney stones, that pain only begins when a stone leaves the kidney and enters the ureter. A stone sitting quietly inside the kidney’s collecting system has no timeline forcing it to move. In one study tracking stones over an average of about three and a half years, roughly 72% of them never caused symptoms at all, and some stayed put for the entire follow-up period.

Why Some Stones Never Leave

The kidney’s internal plumbing is a branching system of cup-shaped chambers called calyces that funnel urine into the renal pelvis and then down through the ureter to the bladder. A stone that forms in one of these chambers can sit there indefinitely if gravity, urine flow, and its own size conspire to keep it in place. There is no muscular contraction inside the kidney actively pushing stones out the way the ureter does with peristalsis. The stone simply has to be carried by the flow of urine into a position where it can slip into the ureter, and for many stones, that never happens.

Location within the kidney matters more than most people realize. A study of 160 stones in 110 patients found that stones in the upper and middle portions of the kidney were far more likely to pass spontaneously than those in the lower pole. Upper and mid-kidney stones passed on their own about 14.5% of the time, while lower-pole stones managed it only about 3% of the time.¹ The lower pole of the kidney is a gravity-dependent pocket. Urine pools there, and a stone sitting at the bottom of that pocket has to travel upward against gravity before it can reach the renal pelvis. Unfavorable anatomy of the lower pole, including a steep angle and a narrow, long drainage channel, creates a trap that makes clearance even harder.²

The Odds of a Stone Becoming a Problem

If you’ve been told you have a kidney stone on imaging but have never felt it, you’re probably wondering whether it will stay silent or eventually cause trouble. The honest answer is that the evidence is frustratingly wide-ranging. A systematic review covering over 25 years of data found that the risk of a small asymptomatic stone eventually causing symptoms ranged from 0% to nearly 60%, depending on the study and the follow-up period.³ That same review found something surprising: the likelihood of developing symptoms was not significantly different between stones smaller than 5 mm and those larger than 10 mm. Size, in other words, is not a reliable predictor of whether a kidney stone will stay quiet.

What does predict trouble is location. That same study of 160 stones found that upper and mid-kidney stones became symptomatic about 41% of the time, compared to about 24% for lower-pole stones.¹ This seems counterintuitive until you think about it: a stone that is more likely to move is also more likely to cause pain on the way out. Lower-pole stones are trapped, which is bad for clearance but good for staying asymptomatic. Among all the stones tracked, about 28% eventually caused symptoms during follow-up averaging around three and a half years.

Stones That Grow While They Wait

A stone sitting in the kidney is not frozen in time. Urine continues to be supersaturated with minerals, and an existing stone acts as a surface for further crystal growth. The process that created the stone in the first place doesn’t stop just because the stone is already there. Most kidney stones in people who form calcium oxalate stones begin on tiny calcium deposits embedded in the tissue of the kidney’s inner surface, called Randall’s plaques. These plaques sit beneath the lining of the kidney, and when the lining erodes, the exposed plaque becomes a seed for crystal buildup from the surrounding urine.⁴ The crystals stack up in layers, alternating between mineral and protein matrix, until eventually a full stone has formed and detached into the collecting system.⁵

That layering process continues after the stone has detached. Even leftover stone fragments after treatment can serve as nuclei for regrowth. A meta-analysis tracking post-treatment fragments found that fragments 4 mm or smaller had a disease progression rate that climbed from about 25% at 20 months to 47% by 50 months. Fragments larger than 4 mm were worse: progression rates rose from 26% at 20 months to 88% by 50 months.⁶ The takeaway is that a small stone left alone in the kidney is not a static problem. It can grow, sometimes substantially, and the longer it sits there, the more likely it is to either cause symptoms or need treatment down the road.

Uric acid crystals can accelerate this process even for calcium-based stones. Laboratory work has shown that uric acid crystals in the urinary tract provoke calcium oxalate crystallization, meaning a uric acid stone or crystal cluster can act as a scaffold for a mixed-composition stone to build on.⁷

What Happens Once a Stone Leaves the Kidney

The moment a stone drops out of the kidney and into the ureter is usually when the real misery begins. The ureter is a muscular tube only a few millimeters wide, and a stone lodged inside it causes intense cramping pain (renal colic) as the ureter squeezes to push it along. How long this takes depends heavily on size.

A classic study of 75 patients with ureteral stones found that stones 2 mm or smaller passed in an average of about 8 days, stones between 2 and 4 mm took about 12 days, and stones 4 mm or larger averaged around 22 days. For 95% of stones to pass, it took about 31 days for the smallest stones and 39 to 40 days for those between 2 and 6 mm.⁸ A larger European study confirmed how steeply the odds drop with increasing width: stones up to 3 mm passed spontaneously about 98% of the time within 20 weeks, while 5 mm stones passed about 65% of the time, and stones 6.5 mm or larger managed it only about 9% of the time.⁹

Smaller stones, more distal location in the ureter (closer to the bladder), and being on the right side were all associated with faster, easier passage.⁸ The right-side advantage is thought to relate to anatomical differences in how the ureters connect to the bladder.

Medications That Help Stones Pass

If a stone has entered the ureter and is expected to pass on its own, doctors sometimes prescribe alpha-blocker medications (most commonly tamsulosin, which many people recognize as a prostate drug) to relax the smooth muscle of the ureter and speed things along. The evidence for this is decent but not overwhelming. A meta-analysis of placebo-controlled trials found that alpha-blockers did not significantly improve overall stone clearance rates across all ureter locations.¹⁰ However, when the analysis was limited to stones in the lower (distal) ureter, alpha-blockers did show a modest benefit. Earlier reviews had been more enthusiastic, noting that alpha-blockers appeared to increase expulsion rates, decrease time to passage, and reduce pain medication needs for distal stones.¹¹

The tradeoff is side effects. That same meta-analysis found alpha-blockers were associated with increased dizziness and, in men, retrograde ejaculation.¹⁰ For a stone already in the distal ureter and small enough to pass, the benefit is real but modest, and the medication makes the most sense when the alternative is surgical intervention.

Active Surveillance and When to Intervene

For a stone still sitting in the kidney and not causing obstruction, both the American Urological Association and the European Association of Urology say that observation is a reasonable option. Both groups acknowledge, though, that the evidence behind this recommendation is thin. The EAU suggests a first follow-up at six months and then yearly imaging to watch for growth or symptoms.¹² The AUA agrees with the general approach but does not specify a timeframe. Neither guideline draws a hard line between “safe to watch” and “needs treatment” based on size alone, which reflects the messy reality of the data.

The decision to intervene on a quiet kidney stone usually comes down to several practical factors: Is the stone growing on serial imaging? Has it started causing intermittent symptoms? Is it in a position where it might eventually obstruct the ureter? Is the patient someone who would be at high risk if an emergency stone event happened, like a pilot, someone who lives far from medical care, or someone with a single functioning kidney? Observation is easier to justify when the stone is small, lower-pole, and the patient has reliable access to follow-up care. It becomes harder to justify when the stone is growing, when the patient has had recurrent infections, or when the stone is in a location where it is likely to drop into the ureter unpredictably.

When a Stone Stays Too Long and Causes Real Damage

Most small stones sitting in the kidney are more of a nuisance than a danger. The real concern is with stones that grow large enough to fill the entire collecting system, forming what urologists call staghorn calculi. These branching stones take the shape of the kidney’s internal drainage system and are associated with serious complications including recurrent urinary tract infections, urosepsis, kidney deterioration, and in the worst cases, death.¹³

A long-term study of 61 patients with staghorn stones managed conservatively (without surgery) found that chronic kidney failure developed in 22 of them, and 7 died from kidney failure. Kidneys that were eventually removed in these patients showed severe damage: swelling from backed-up urine, abscesses, and a destructive inflammatory condition. The morbidity and mortality rates were substantially higher in the conservatively managed group compared to those who had surgery.¹⁴ Staghorn stones represent the extreme end of what happens when a kidney stone stays in the kidney and keeps growing. They are the strongest argument against ignoring a growing stone, even if it has not yet caused pain.

Even non-staghorn stones can damage the kidney if they silently obstruct the ureter. A study of patients with “silent” ureteral stones, stones blocking the ureter without causing obvious symptoms, found that the affected kidneys had notably reduced function on nuclear medicine scans, averaging about 31% of total kidney function after treatment.¹⁵ Some of that damage was irreversible. The absence of pain does not mean the absence of harm.

The Imaging Challenge of Watching a Stone

If your doctor recommends surveillance, the imaging tool used to track the stone matters more than you might expect. Ultrasound is the default for follow-up because it involves no radiation, but it has a well-documented tendency to overestimate stone size, particularly for small stones at or below 5 mm.¹⁶ A stone measured at 6 mm on ultrasound might actually be 4 mm on CT, which could change the entire treatment plan.

CT scans are far more accurate for sizing and locating stones, and recent advances in ultra-low-dose CT protocols have reduced radiation exposure by about two-thirds compared to standard low-dose CT, bringing the dose down to roughly 1.7 mSv. A quality-improvement study found that adopting ultra-low-dose CT for stone surveillance reduced reliance on ultrasound and plain X-rays without meaningfully increasing radiation risk.¹⁷ If you are being followed for a kidney stone and your management hinges on whether the stone is growing, it is worth asking whether a low-dose CT would give better information than the ultrasound your doctor may reflexively order.

Kidney Stones During Pregnancy

Pregnancy changes the kidney stone equation in several ways. The hormonal and anatomical changes of pregnancy cause the ureters to dilate, and the increased blood flow through the kidneys raises urine output. Stone episodes in pregnant women cluster in the second and third trimesters, with about 39% and 46% of episodes occurring in those periods respectively. The good news is that spontaneous passage rates during pregnancy are much higher than in non-pregnant women: about 81% versus 47%.¹⁸ The dilated ureters give stones more room to pass on their own.

The flip side is that treatment options are limited during pregnancy. Radiation from CT scans is avoided, and surgical interventions carry added risk. Most pregnant women with kidney stones are managed with pain control and hydration while waiting for spontaneous passage. The high passage rate is reassuring, but any stone that doesn’t pass and causes obstruction during pregnancy requires careful coordination between urology and obstetrics.

Kidney Stones in Space

Astronauts face an unusually high risk of kidney stones, despite being screened for stone history before selection. Among 357 astronauts studied, 22 experienced at least one stone episode, with 36 total episodes recorded. The reasons stack up: microgravity causes bone mineral loss, which dumps calcium into the urine. Astronauts tend to drink less water than they should. Their diets run high in acid-producing foods, which pushes urine pH down and reduces protective citrate levels.¹⁹ Laboratory experiments in simulated microgravity have shown that certain microorganisms thought to be involved in stone formation multiply several times faster than in normal gravity, and the mineral crystals they produce differ depending on the culture conditions.²⁰

Most astronauts’ stones appear after returning to Earth rather than during the mission itself, which suggests that the biochemical changes accumulate during flight but the actual stone event is triggered or noticed once normal gravity resumes. For long-duration missions to Mars or beyond, kidney stone prevention is a genuine concern for space agencies, since surgical capability in space is essentially nonexistent. Current countermeasures focus on hydration protocols, dietary management, and potassium citrate supplementation to keep urine chemistry in a safer range.