Adenoid cystic/basal cell carcinoma (ACBCC) of the prostate has been considered to have indolent biologic potential. However, outcome data are scant, with only one documented metastasis and death. We describe clinicopathologic features of ACBCC in 19 patients and document outcome in 15. Patients ranged in age from 43 to 83 years. All but one presented with urinary obstruction. ACBCC was diagnosed by transurethral resection in 15 cases, by needle biopsy in 3 cases, and unexpected in 1 case. Four patients had concurrent acinar adenocarcinoma. Histologically, cribriform or adenoid cystic patterns predominated in 12 cases and basal cell carcinoma pattern in 7. Five cases had prominent perineural invasion. ACBCC was immunoreactive for p63 and cytokeratins 7 and 34 beta E12 but not cytokeratin 20. After diagnosis, 5 patients underwent radical prostatectomy, 2 underwent pelvic exenteration, and the rest had no treatment. ACBCC showed extraprostatic extension in 5 cases and involved the bladder margin in 3. Metastases developed in 4 (21%) patients: liver (2), lung (2), bowel (1), and corpus cavernosum (1). In 15 cases with follow-up (0.3-11.8 years), two patients died of cancer (at 1.5 and 3 years after diagnosis), 3 remain alive with cancer, and 10 have no evidence of cancer. Thus, ACBCC of the prostate is a potentially aggressive neoplasm requiring ablative therapy.