Kidney pain most commonly results from kidney stones, infections, or swelling caused by a blockage in the urinary tract. It’s felt in the flank, the area on either side of the spine just below the rib cage and above the hips. Unlike muscular back pain, kidney pain typically doesn’t improve or worsen with movement, and it generally won’t resolve on its own without treatment.
Understanding the specific cause matters because the type of pain, how quickly it develops, and the symptoms that accompany it all point toward different conditions, some urgent and some manageable at home.
How Kidney Pain Differs From Back Pain
The kidneys sit against the back muscles just below the rib cage, which is why kidney problems are so easily confused with a pulled muscle or spinal issue. The distinction comes down to a few reliable patterns. Muscular back pain tends to feel like a dull ache, stiffness, or soreness that changes when you shift position. It often improves when you find a comfortable way to sit or lie down, and it may radiate down the legs if a nerve is involved.
Kidney pain behaves differently. It stays in the flank area or spreads to the lower abdomen and inner thighs rather than the legs. Moving around doesn’t help or make it worse. It remains constant or comes in waves, and it persists until the underlying problem is addressed. The pain itself comes from stretching or inflammation of the kidney’s outer capsule, a thin membrane packed with nerve endings that responds to any swelling or pressure inside the organ.
Kidney Stones
Kidney stones are the most well-known cause of acute kidney pain, and the experience is notoriously intense. A stone sitting quietly inside the kidney usually causes no symptoms at all. The pain begins when a stone moves into one of the ureters, the narrow tubes connecting each kidney to the bladder. If a stone gets stuck there, it blocks urine flow, causing the kidney to swell and the ureter to spasm. That combination produces what’s called renal colic: serious, sharp pain in the side and back below the ribs that can spread to the lower abdomen and groin.
The pain typically comes in waves, varying in intensity as the ureter contracts trying to push the stone through. As the stone moves through the urinary tract, the pain often shifts location and may change in character. You might also notice blood in your urine, nausea, or a persistent urge to urinate. Kidney stones affect roughly 1% of privately insured adults under 65 in any given year, with the rate climbing to about 4% in adults over 65.
Kidney Infections
A kidney infection, known medically as pyelonephritis, typically starts as a lower urinary tract infection that travels upward. It causes a distinct combination of symptoms: sudden fever, chills, severe flank pain, nausea and vomiting, and tenderness when the area just below the ribs on your back is pressed. You’ll often notice burning during urination, an urgent or frequent need to go, or both.
What sets infection pain apart from stone pain is the fever and the general feeling of illness that comes with it. Stone pain can be excruciating, but you usually don’t feel systemically sick the way you do with a kidney infection. The flank pain from an infection also tends to be more constant rather than coming in waves. Kidney infections require prompt treatment with antibiotics because the infection can spread to the bloodstream if left unchecked.
Urinary Tract Blockages
Any obstruction that prevents urine from draining out of the kidney causes it to swell, a condition called hydronephrosis. Kidney stones are the most common culprit, but blockages can also come from enlarged prostate tissue, scar tissue in the ureter, or, less commonly, tumors pressing on the urinary tract from outside.
The pain from hydronephrosis can range from sudden and intense to a slow-building ache, depending on how quickly the blockage develops. A stone that lodges suddenly produces sharp, immediate pain. A tumor gradually compressing a ureter may cause a dull discomfort that worsens over weeks. Other symptoms include blood in the urine, painful urination, and sometimes weakness or fever if an infection develops behind the blockage.
Polycystic Kidney Disease
Polycystic kidney disease (PKD) is an inherited condition in which fluid-filled cysts grow throughout the kidneys, gradually enlarging them. Pain is common in PKD and typically felt in the side or back. It can come and go or become a chronic, ongoing problem as the kidneys grow larger and their outer capsule stretches.
Several things can trigger sharper episodes of pain in PKD: bleeding inside a cyst, a urinary tract infection, or a kidney stone forming among the cysts. The enlarged kidneys in PKD are also more vulnerable to physical trauma. Because the pain has multiple possible sources, a new or worsening pain pattern in someone with PKD usually warrants investigation to identify the specific trigger.
Blood Flow Problems
A less common but serious cause of kidney pain is renal infarction, which happens when a blood clot blocks the artery supplying the kidney. This cuts off blood flow and causes tissue damage, similar to what happens during a heart attack but in the kidney. It produces acute flank or abdominal pain in about 65% of cases, along with nausea and vomiting (20%), fever, and blood in the urine.
Renal infarction often mimics kidney stones or other more common problems, which can delay diagnosis. People with a history of blood-clotting disorders or atrial fibrillation (an irregular heart rhythm that can form clots) face a higher risk. The key difference from stone pain is that it tends to be constant rather than wave-like, and standard imaging for stones may not reveal it. Diagnosis often requires specific blood tests and imaging that evaluates blood flow rather than anatomy.
Kidney Tumors
Kidney cancer is a relatively uncommon cause of kidney pain. Only about 10% of people with kidney cancer report flank or abdominal pain as a symptom. When pain does occur, it’s usually described as a dull, persistent ache that worsens over time rather than a sudden, sharp event. Most kidney tumors grow slowly without stretching the kidney’s capsule enough to trigger pain.
The exception is when sudden bleeding occurs within a tumor, quickly expanding the kidney capsule and producing acute pain. If a kidney tumor has spread to other organs, particularly bone, it can cause bone pain at distant sites. Most kidney cancers today are found incidentally on imaging done for other reasons rather than because of pain.
Warning Signs That Need Urgent Attention
Some patterns of kidney pain signal a situation that needs prompt medical evaluation. These include pain you can’t manage with over-the-counter medication, inability to urinate, fever or chills alongside flank pain, severe nausea or vomiting (especially if you can’t keep fluids down), visible blood in your urine, or a repeated urgent need to urinate that’s unusual for you. A general feeling of illness or persistent low energy combined with back or side pain also warrants evaluation.
Fever with flank pain is particularly important because it may indicate an infection that could worsen quickly. Inability to urinate suggests a complete blockage that can damage the kidney if not relieved. And persistent, unexplained flank pain that doesn’t fit the pattern of a muscle strain deserves investigation even without dramatic symptoms, since some serious causes like renal infarction or slowly growing tumors present subtly.