Multiple independent tumors were identified in specimens from 117 of 234 prostatectomies for clinical adenocarcinoma; there were 266 incidental cancers in these 117 prostates. The clinically detected carcinoma was the largest (or only) tumor in all 202 Stage B cases. However, among 32 Stage A cases (detection by transurethral resection), there were 8 prostates in which an incidental tumor was larger than the clinically manifest cancer. These were all small tumors except for two incidental cancers with a volume greater than 2 cm³; roughly 80% of incidental carcinomas were smaller than 0.5 cm³, whereas fewer than 20% of manifest tumors were smaller than 0.5 cm³. Comparison with a series of cancers found incidentally at cystoprostatectomy for bladder cancer showed the same volume distribution as incidental (smaller) carcinomas in patients with prostate cancer. This distribution was thought to reflect the volume distribution of prostate cancer in the general population older than 50 years of age. It was concluded that additional incidental tumors are common in patients with prostate cancer, but their sum of volumes is seldom as large as the clinical cancer volume. Cancer 1992; 70:2313‐2318.