The radiodiagnostic difficulties of renal hamartoma (renal angiomyolipoma) have been discussed in connection with the authors' two cases. In the first case a unifocal cystic renal angiomyolipoma without tuberous sclerosis caused retroperitoneal haematoma and acute symptoms. In the second case a preoperative tentative diagnosis of angiomyolipoma was made, however, the Pringle-type dermal lesions were only observed 6 months after surgery on control examination. The authors state that the radiographic functional findings vary depending on the tissue structure of the tumour. Differential diagnosis of cystic angiomyolipoma containing little fatty and angiomatous tissue is impossible, whereas for the hypervascular variant a tentative diagnosis can be established even in the case of unifocal hamartoma.