The authors identified incidental adenocarcinoma of the prostate in serial cross-sections of prostates from 61 of 100 patients having cystoprostatectomy for bladder cancer. In 20 patients, cancer foci were in the prostatic region accessible to transurethral resection (TUR). Eight of these 20 patients had stage A1 disease (less than or equal to 3 foci); in six of these patients all foci were located at or within the transition-zone boundary only. Twelve patients had stage A2 disease (greater than 3 foci); in none were more than three foci at or within the transition-zone boundary. Among the 20 patients, 84 tumor foci were distributed as follows: 62% in the peripheral zone, 10% in the transition zone, 3% in the central zone, and 25% in the transition zone boundary. The authors' data suggest that Stage A prostate cancer, like larger clinically detectable cancers, is multifocal, multizonal, and similarly distributed with respect to the urethra. No case of Stage A1 disease would have been reclassified as Stage A2 by repeat TUR of the entire transition zone and transition zone boundary. The authors' results confirm those of previous studies that report a high incidence of residual carcinoma in regions not accessible to TUR in patients with presumptive Stage A1 disease and suggest that ultrasonography and ultrasonographically guided biopsies, rather than repeat TUR, may more accurately define the extent of Stage A disease in some patients.