No data exist on urothelial carcinoma diagnosed on prostatic needle biopsy. We reviewed 21 cases (19 consultations) of urothelial carcinoma diagnosed on prostate needle biopsy from 1991 to 1998. In 13 of 21 (62%) cases, urothelial carcinoma showed in situ urothelial carcinoma involving prostatic ducts and acini (DCIS) only; 6 of 21 (29%) cases showed both DCIS and invasive carcinoma and 2 of 21 (9%) cases showed widespread stromal invasion without DCIS. In contrast to prostatic adenocarcinoma, cases exhibited greater nuclear pleomorphism, variably prominent nucleoli, increased mitoses, and necrosis. Immunostains for PSA and PSAP were negative in all 18 cases studied. CK7 was positive in 14 of 16 cases, CK20 was positive in 13 of 16 cases, and 34betaE12 was positive in 11 of 17 cases. A total of 7 of 17 (41%) men had no prior or subsequent history of urothelial carcinoma outside the prostate, 6 of 17 (35%) had concurrent urothelial cell carcinomas of the bladder (1 with extensive carcinoma in situ [CIS] at cystoprostatectomy), 2 of 17 (12%) had a prior urothelial cell carcinoma, and 2 of 17 (12%) developed urothelial cell carcinomas outside the prostate subsequent to the needle biopsy diagnosis. A total of 14 of 18 (78%) men had an elevated prostate specific antigen (PSA), abnormal digital rectal examination, or abnormal ultrasound suggestive of prostatic adenocarcinoma. Follow-up information was available in 17 cases. Six of nine (67%) patients with DCIS eventually died of disease (DOD) (2 with prior urothelial cell carcinoma, 1 with no prior or subsequent history, 3 without information), and 3 of 9 (33%) patients with DCIS were alive with residual disease (AWD). Of the patients with invasive carcinomas, 4 of 8 (50%) were DOD, 2 of 8 (25%) were AWD, and 2 of 8 (25%) were alive without evidence of disease. All men who are alive were treated aggressively with surgery and often adjuvant chemotherapy-radiation. Overall, 10 of 17 (59%) men were DOD with a mean survival after diagnosis of 23.2 months (2-72 months). The diagnosis of urothelial carcinoma on prostate needle biopsy is difficult because it is rare and clinically can mimic prostatic adenocarcinoma; often there is no history of urothelial carcinoma elsewhere. Although the prognosis is poor even with only apparent DCIS, histologic recognition is essential because the only opportunity for improved outcome is early and aggressive treatment.