We report an uncommon case of primary prostatic signet-ring cell carcinoma which meets all criteria that define this clinicopathologic entity. Histologically, the tumor showed three different growth patterns, all of which contained large numbers of signet-ring cells. The predominant pattern, comprising approximately 50 percent of the tumor, was solid sheets of pure signet-ring cells. An intriguing finding was the presence of intestinal metaplasia involving the prostatic urethra and the large periurethral ducts. All mucin stains were intensely positive within the signet-ring cells and in the mucin lakes. Signet-ring cells stained positively for prostatic specific antigen, prostatic acid phosphatase, and carcinoembryonic antigen immunoperoxidase markers. Our patient presented with symptoms of urinary tract obstruction and locally widespread disease, infiltrating the rectum and the bladder, thus demonstrating the aggressive biologic behavior that traditionally has been ascribed to signet-ring cell carcinomas.