In this paper the morphologic characteristics of renal adenoma are studied in 4 groups of kidneys from 500 unselected autopsies performed for legal reasons, 5,000 autopsies from the files of a general hospital, 16 kidneys with adenoma and without carcinoma, and 200 kidneys with carcinoma. All the tumors were classified by pattern, cytology and sclerosis of the renal nontumoral stroma. The relationship between tumor size, tumor histological type and sclerosis was evaluated by statistical analysis. The differences in frequency were significant between the parameters size and histological type (p < 0.003), namely for solitary adenomas (p < 0.001). The differences in histological type observed in multiple adenomas were not significant (p > 0.05). The differences were significant between the extent of sclerosis and histological type (p < 0.002) and tumor size (p < 0.009). This study identifies two distinct groups of adenomas according to size, histological type and extent of sclerosis, and suggests that small adenomas of the mixed tubulopapillary type and with basophilic cells have little or no malignant potential. Conversely, adenomas with a solid or papillary pattern and clear cells are potentially malignant.