A 65-year-old man with pyuria visited our hospital. Physical examination had revealed a mass in the left upper quadrant of the abdomen. Intravenous pyelogram, ultrasonogram, computerized tomography and selective renal arteriogram suggested a hypervascular renal cell carcinoma on the left side of the horseshoe kidney. A left radical nephrectomy with division of the isthmus was performed. The pathological diagnosis indicated renal cell carcinoma consisting of clear cell type without invasion of the capsule or renal pelvis. Treatment with alpha-interferon was started and has continued for six months with no evidence of recurrence. Only 35 cases of horseshoe kidney with a renal tumor have been reported in the Japanese literature. Among them, adenocarcinoma was present in 54.3% of the cases, renal pelvic tumor in 17.1%, and nephroblastoma in 14.3%. While adenocarcinoma in horseshoe kidney is seen less often than normal kidney, there is an increased incidence of both renal pelvic tumor and nephroblastoma.