Small cell carcinomas of the prostate are rare. A few reported cases have manifested morphologic and functional neuroendocrine characteristics, and it has been suggested that these tumors are derived from the argentaffinic/argyrophilic cells normally present in the prostate. The authors have recently studied three cases of primary prostatic small cell carcinoma in which the small cell component developed during the course of progression of "regular" prostatic adenocarcinoma, and reflected a terminal aggressive phase of the disease. Immunoperoxidase staining for prostate-specific acid phosphatase (PSAP) showed positivity in the adenocarcinoma but absence in the small cell component of each tumor. The association of small cell carcinoma with prostatic adenocarcinoma indicates that in considering the histogenesis of prostatic small cell carcinoma, a specific neuroendocrine cell of origin need not be implicated.