Occult hydronephrosis may commonly manifest symptoms following even minor blunt trauma. The usual presentation is pain and gross hematuria. However pelvic or caliceal rupture can occur with extravasation into the retroperitoneum. Under extremely rare circumstances free intraperitoneal rupture occurs and may present as an acute abdomen. CASE REPORT A 64-year-old Asian woman fell on the right side. She was evaluated elsewhere and noted to be alert, oriented, hypertensive and hemodynamically stable. She had a history of hypertension but was taking no medications. She denied any other medical or surgical history. Examination was significant for abdominal tenderness and guarding. Abdominal computerized tomography (CT) (no contrast medium reported) demonstrated injuries consistent with traumatic renal fracture. At transfer to our institution the patient was in significant distress secondary to diffuse abdominal pain. Blood pressure was 190/100 mm. Hg with heart rate of 90 to 100 per minute. Mild contusions involved the right hand, forearm and flank. The abdomen was somewhat distended with marked diffuse tenderness, worse on the right than the leR side. Involuntary guarding was consistent with peritoneal irritation and the right costovertebral angle was moderately tender. Rectal examination revealed guaiac positive stool but the remainder of the cardiac, pulmonary, vascular and neurological examination was normal. Urine was grossly clear. Laboratory values included white blood count 20 x 103/pl. (normal 4.3 to 10.81, hematocrit 40% (normal 37 to 48), normal electrolytes, creatinine 1.2 mg./dl. (normal 0.6 to 1.51, and urinalysis 3 to 5 red and 0 to 2 white blood cells per high power field. Chest x-ray demonstrated clear lungs and mild cardiomegaly without pneumothorax or rib fracture. Review of the unenhanced CT demonstrated a heterogeneous fluid density filling a large multicystic structure in the right retroperitoneum. Normal kidney parenchyma was not identified. The examination was rzpeated with intravenous contrast medium using a standard helical CT trauma protocol. Findings were notable for a multicystic structure with thin peripheral enhancement and heterogeneous density in some areas suggestive of hemorrhage (part A of figure). In a lower cut a 1.5 cm. calcification was noted just inferior to the