Spontaneous subcapsular or perinephric hematoma in the absence of anticoagulation, arteritis or trauma is most likely due to an underlying renal tumor. Eight such patients recently have been evaluated and after nephrectomy 5 had small tumors undetectable by imaging techniques, including computerized tomography or angiography. In 1 patient a tumor was demonstrated preoperatively by angiography and in only 2 was a tumor not found. In the absence of an apparent etiology, patients with spontaneous renal bleeding and a normal contralateral kidney should undergo radical nephrectomy because of the extremely high incidence of small undetectable occult tumors, often less than 2 cm. in size.