Intraperitoneal rupture of hydronephrosis is rare. We report a case of hydronephrosis caused by ureteral cancer accompanied by ipsilateral renal cell carcinoma. CASE REPORT A 72-year-old man complaining of general weakness and a tumor in the left upper abdomen without tenderness presented at our hospital. Abdominal ultrasonography and computerized tomography (CT) revealed a giant renal cystic mass and a solid mass at the lower end of the ureter (fig. 1, a to d). Diagnosis was tumor of the left ureteral stump, and a series of operations was scheduled. However, the patient complained of sudden reduction of the mass and resolution of the sensation of abdominal fullness. CT revealed a marked decrease in hydronephrosis and a large amount of ascites in the peritoneal cavity (fig. 1, e and f). Intraperitoneal rupture of left hydronephrosis was diagnosed, and an emergency procedure was performed. When the intraperitoneal cavity was entered through a midline incision there was approximately 1,000 ml. of dark brown ascites and an approximately 5 mm. foramen, from which bloody liquid was excreted into the descending colonic mesentery. Total nephroureterectomy was performed. Extirpated weight was 2,900 g. (including 1,900 ml. of dark fluid). There was a large amount of clotted blood within the renal pelvis, which revealed marked wall thickening. Papillary tumors were noted along the entire ureter. Cytological evaluation of the ascites and renal pelvic urine revealed class IV. Pathological diagnosis regarding the ureter was transitional cell carcinoma, grade 2 greater than 1, pT1, interferon-, pR0, pN0. An induration 3.5 cm. in diameter (fig. 2, a) was noted in the totally destroyed renal tissue (fig. 2, b). Pathological diagnosis was renal cell carcinoma, clear cell subtype, grade 2, interferon- ,V (), pT1a, pN0. Also, the ruptured portion of the kidney comprised necrotic