A case of simultaneous double malignant tumor in the same kidney, associating renal cell carcinoma with renal pelvic transitional cell carcinoma, in a 70 year-old-male was reported. On January 6, 2000 he presented with macroscopic hematuria. There were no remarkable findings on cystoscopic examination. Drip infusion pyelography and multidetector-row computed tomography demonstrated a tumor mass on the upper pole of the left kidney and atrophic right kidney. Systemic chemotherapy with CDDP, MTX and ADR was performed preoperatively. Then, hemi-left nephrectomy underwent with the diagnosis of renal pelvic tumor and renal tumor. The surgical specimen was pathologically diagnosed as transitional cell carcinoma of the renal pelvis and renal cell carcinoma of its upper pole. This is the 32nd case of simultaneous occurrence of renal cell carcinoma and transitional cell carcinoma in the same kidney in the Japanese literature.