The early v late occurrence of tubular epithelial hyperplasia and the frequency and malignant potential of renal neoplasms in autosomal dominant polycystic kidney disease (ADPKD) are controversial. The kidneys from 87 patients with documented or presumed ADPKD, removed at autopsy (n = 49) or prior to transplantation (n = 38), were thoroughly sectioned and examined. Hyperplastic polyps were found in 90.8% of the patients, even in the absence of renal insufficiency or marked renal enlargement. However, their number was significantly higher in the patients with advanced stages of the disease, and especially in those with a history of dialysis. Hyperplastic polyps were not detected in eight cases, seven of which had no evidence of epithelial hyperplasia, despite thorough examination of multiple sections. A total of 42 neoplasms were observed in 24.1% of the patients. One patient had bilateral low-grade clear cell adenocarcinoma. Another patient had a transitional cell neoplasm. The remaining 39 neoplasms were microscopic adenomas. Neoplasms tended to occur more often in men and older patients. None of these neoplasms had been clinically diagnosed, and no metastasis had occurred. Nonneoplastic mass lesions were observed in two patients: one had malacoplakia and the other had xanthogranulomatous pyelonephritis.