Prostate glands from 150 patients with carcinoma of the bilharzial bladder who underwent cystoprostatectomy were studied histopathologically by step sections. Prostatic urethral involvement by urothelial carcinoma was noted in 13 out of 96 (13.5%) and 5 out of 40 (12.5%) squamous and transitional cell tumors, respectively. None of the 12 adenocarcinomas and the two undifferentiated tumors showed involvement. Prostatic urethral involvement was as high as 19% in basal tumors and 26.7% in multifocal tumors compared to only 6.5% when the tumors occupied the bladder body. There was a significant increase in the incidence of prostatic urethral involvement from 9.5 to 35% when the prostate gland was involved. Prostate gland was involved in 20 out of 150 (13.3%). The bladder tumor was basal and infiltrating the prostate in 18 such cases. Seminal vesicles were infiltrated in 6 cases from the adjacent basal bladder tumors. We conclude that patients with basal or multifocal tumors are risky regarding bladder substitution and we recommend routine diagnostic transurethral prostatic biopsies and frozen sections from the site of urethral transection during cystoprostatectomy whenever bladder substitution controlled by the urethral sphincter is considered.