Pain associated with the kidney is the result of sudden distention of the renal capsule as occurs with acute ureteral obstruction. It is referred to as flank pain and is usually colicky in nature (i.e., intermittent or in waves). Patients are often restless and cannot find a comfortable position. Reflex nausea and vomiting may be noted because of the common autonomic and sensory innervation of the gastrointestinal and urologic systems.

A dull, constant ache in the costovertebral angle can also characterize renal pain. This is less likely to be associated with acute obstruction but rather is secondary to renal parenchymal enlargement from pyelonephritis or a tumor. Renal pain must not be confused with flank or back pain of musculoskeletal origin or radiculitis, which can be aggravated or relieved by postural changes. Finally, remember that many renal diseases are painless, despite massive degrees of obstruction or kidney enlargement by tumors, because of the slow, gradual progression of the disease.