A 57-year-old man was admitted to our hospital complaining of nausea, vomiting and fever of 38.7 degrees C. He was diagnosed as having acute cholecystitis with gallstones. Abdominal CT, however, incidentally revealed a space-occupying solid mass lesion at the upper pole of the left kidney. The feature of the lesion on ultrasonography was similar to that of renal simple cyst. The renal angiography showed that the tumor was avascular. Aspiration biopsy was done. Cytologically, small tumor cells forming cell clusters had scanty granular cytoplasms and small round or oval shaped nuclei sized 13-15 mu. The chromatin was diffusely distributed and increasing its density. Nucleoli were not so evident and if existing, usually small. Fatty stain was positive at granules in the cytoplasms. Radical nephrectomy was performed on August 28, 1984. Pathological examination revealed that almost all components of the tumor consisted of typical papillary renal adenocarcinoma, and staging was pT2, pN0, pV0, M0, INF alpha. Alpha-type interferon to a total doze of 11,700 X 10(4) units was administered intramuscularly daily for a month after the operation. By January 11, 1986, no evidence of tumor recurrence was noted.