Primary small cell carcinoma of the renal pelvis is rare. We report a case of combined small cell and transitional cell carcinoma of the renal pelvis. The patient was a 78-year-old man with macrohematuria. He was diagnosed with right pelvic tumor by right retrograde pyelography and computerized tomography. A right radical nephroureterectomy was performed. Histological sections of the tumor showed a mixture of small cell and grade 2 transitional cell carcinoma. Positivity for Grimelius was noted in the small cell component. Masson-Fontana, chromogranin A, serotonin, keratin, leucocyte common antigen (LCA), neuron specific enolase (NSE), adrenocorticotropic hormone (ACTH), growth hormone (GH) and somatostatin were negative. Ultrastructurally, the tumor cells were tightly packed and attached together by scattered desmosomes. There were no neurosecretory granules. Seven months after operation, the patient died with peritonitis carcinomatosa. At autopsy, a metastatic tumor was found in the liver and retroperitoneal lymph node. Other organs were not involved. Especially, there were no abnormalities in the lungs.