We report on a case of renal cell carcinoma in a functionally solitary kidney, treated by ex vivo partial nephrectomy and autotransplantation. A 62-year-old woman was admitted for examination of a left renal mass and right hydronephrosis, incidentally detected on echography and CT scan. The right hydronephrosis was caused by congenital ureteropelvic junction obstruction, associated with severely impaired renal function. A solid tumor was found in the left kidney, located in the upper pole and extending to the hilus, and was suspected to be renal cell carcinoma. The patient was treated by ex vivo partial nephrectomy of the left kidney followed by autotransplantation. The tumor was 35 x 55 x 40 mm in size and diagnosed histologically to be renal cell carcinoma. Hemodialysis was unnecessary since good urine output was obtained postoperatively from the transplanted kidney. The patient was discharged 3 months after surgery, following interferon-alpha therapy, with serum creatinine level of 1.4 mg/dl.