Clinical, pathologic, and ultrastructural features of two cases of adult nephroblastoma (Wilms' tumor) are presented, one in a 77-year-old man who had a large tumor with extracapsular extension and blood vessel invasion. Histologically, it consisted mainly of blastema. The patient died during the first course of postoperative chemotherapy with dactinomycin. The second patient was a 47-year-old woman with a tumor limited to the renal parenchyma. Histologically, the tumor was mainly epithelial. This tumor was discovered incidentally and remained unchanged for 5 years before surgery. The patient underwent nephrectomy and refused further treatment. She was free of disease 2 years after surgery.