Twelve patients with primary mucinous adenocarcinoma of the prostate were included in a clinicopathologic study; criteria included a total tumor volume more than 25% mucinous and single or clustered tumor cells floating in mucin lakes. Patient ages were 57 to 81 years; tumor stages were C (three), D (five), and unknown (four). Bone was the most frequent metastatic site (usually osteoblastic), followed by lymph nodes and lungs. Serum levels of prostatic acid phosphatase and prostate-specific antigen were frequently elevated (five of 10 and three of three measured, respectively). All mucinous adenocarcinomas also contained other histologic patterns: microglandular (four), cribriform (three), comedo (two), solid (two), and hypernephroid (one). Mucinous components composed less than 50% of three tumors, 50% and 75% of six, and more than 75% of three. No tumor contained signet-ring cells. Immunoperoxidase staining was positive for prostatic acid phosphatase and prostate-specific antigen and negative for carcinoembryonic antigen. Treatment was radiation, estrogen, orchiectomy, or a combination. In two of four patients, serum prostatic acid phosphatase levels normalized after therapy. Seven patients died of disease (mean follow-up, 56 months), and five patients are alive with disease (mean, 32.2 months). The proportion of mucinous component did not affect prognosis.