A five year experience with spontaneous perirenal hemorrhage at the Brigham and Women's Hospital was reviewed, with 15 instances identified. Ages ranged from 17 to 80 years (mean of 56 years). Abdominal or flank pain was the presenting symptom in 13 patients; one patient was in a state of shock. Anemia and elevation of lactate dehydrogenase were uniformly present during acute evaluation. Computerized tomography (CT) identified lesions other than hematoma in ten of 14 instances. Arteriography was essential to the diagnosis of two vascular lesions not appreciated by other imaging modalities. Intravenous pyelography and ultrasound did not add significantly to findings on CT or arteriography. Six patients underwent serial CT evaluation; three with persistent nonfatty lesions had carcinoma of the kidney confirmed at operation. Carcinoma of the kidney occurred in a total of eight patients and angiomyolipoma in three patients. It was suggested that patients with clinical evidence for spontaneous perirenal hemorrhage should be evaluated by CT, with arteriography added when the underlying cause remains uncertain. Nephrectomy should be performed for CT evidence of nonfatty lesions other than hematoma. Other patients may have serial CT with nephrectomy reserved for persistent abnormalities. The protocol decreases the likelihood of nephrectomy for benign disease while addressing the high incidence of carcinoma of the kidney among patients with spontaneous perirenal hemorrhage.