The distribution of residual cancer in radical prostatectomy specimens in stage A prostate cancer. To assess the volume and distribution of residual cancer after transurethral resection of the prostate in stage A cancer patients 42 step-sectioned radical prostatectomy specimens were examined, and the volume, location, grade and extracapsular extension of the residual tumor were recorded. A total of 13 patients had stage A1 tumors (5% or less tumor in the transurethral resection specimen and a Gleason sum of 7 or less) and 29 had stage A2 disease. Residual cancer was present in the radical prostatectomy specimen in 41 patients (98%) with a mean volume of 1.28 cc. The location of residual cancer, that is multifocal (76%), peripheral (81%) and distal to the verumontanum (66%), makes complete removal or even identification of residual tumor (restaging) by repeat transurethral resection improbable. Of the stage A1 cancer patients 4 (30%) had more than 1 cc residual tumor volume, extracapsular extension or seminal vesicle invasion. On the other hand, 14 of the stage A2 cancer patients (48%) had less than 1 cc residual tumor completely confined to the gland. Foci of residual cancer were found in the transition zone in 67% and in the peripheral zone in 90% of the patients. The grade of the residual peripheral zone cancer was significantly higher than that of the transition zone cancer in the same gland (p = 0.0004). Eight of 13 instances of extracapsular extension and all 5 of seminal vesicle invasion were directly attributable to peripheral zone cancer. These observations imply that the greatest threat to patients with stage A prostate cancer may be a separate, associated cancer in the peripheral zone rather than the primary transition zone cancer incidentally removed at transurethral resection. 