Signet-ring carcinoma is a rare variant of prostatic adenocarcinoma with grim prognosis. It is generally agreed that at least 25% of the tumor should be composed of signet-ring cells for this classification. Most patients present at advanced clinical stages. The tumor cells are arranged in sheets and strands and show prominent cytoplasmic vacuoles pushing the nucleus aside. Signet-ring cell carcinoma is occasionally detected in needle biopsy specimens. Most cases are admixed with usual acinar adenocarcinoma elsewhere in the gland. Although signet-ring cell variant was not included in the original Gleason grading system, most cases are assigned Gleason scores of 5+5 or 5+4 in keeping with their biologic behavior.