We report 50 examples of an uncommon type of renal adenoma from the files of the Armed Forces Institute of Pathology. They appear to be benign tumors with no malignant potential, and their chief importance is related to the fact that they are most often misinterpreted as renal cell carcinoma or epithelial Wilms' tumor. They predominated in females by well over 2:1. The mean age of the patients was 41 years, with a range of 5 to 83 years, and the mean size was 5.5 cm, with a range of 0.3 to 15.0 cm. Presenting signs and symptoms included pain in 11, hematuria in five, and palpable mass in five. In 20 patients the tumors were found incidentally during evaluation for other problems, and in six the other problem was polycythemia. This finding establishes a higher incidence of polycythemia in renal adenoma than in other previously reported renal diseases. Also of preoperative importance is the fact that these tumors are more commonly calcified than other renal neoplasms. Microscopically, these tumors consist of very small epithelial cells that form very small acini in an acellular stroma. Less often, they form tubular, glomeruloid, or polypoid and papillary formations. Most also show evidence of regression in the form of scarring and calcification. These lesions seem histogenetically related to epithelial Wilms' tumor, and, in fact, the two may occur together. They are histologically very similar to the metanephric hamartomatous element of nephroblastomatosis.