Neuroendocrine differentiation in prostatic neoplasms has in the past been considered extremely uncommon. The histologic neuroendocrine patterns reported previously vary from small cell to carcinoidlike to mixed adenocarcinoma--small cell or carcinoid. The majority of the tumors reported are of the mixed variety. We reviewed 2648 autopsies, revealing 69 prostatic carcinomas, eight with neuroendocrine differentiation (five mixed adenocarcinoma--small-cell carcinoma, two "pure" small cell, and one "pure" carcinoidlike). The mean patient age was 69.5 years. One patient presented with markedly elevated serum corticotropin and another was severely hypercalcemic with elevated serum parathyroid hormone level. Three neoplasms were incidental autopsy findings. The mean survival time, after diagnosis, was 19 months for the other patients. Three of the cases were examined ultrastructurally and showed cytoplasmic processes containing membrane-bound granules in the neuroendocrine component. The areas with neuroendocrine differentiation were positive for markers as follows: neuron-specific enolase, seven of eight; prostate-specific antigen (PSA), none of eight; chromogranin A, seven of eight; synaptophysin, four of eight; and calcitonin, four of eight. Those neoplasms mixed with an adenocarcinoma component showed well-defined PSA positivity in the glandular elements. This study suggests that neuroendocrine differentiation in prostatic neoplasms may be more common than previously thought. Often, the areas with neuroendocrine differentiation are considered to represent poorly differentiated adenocarcinoma. It is important to recognize neuroendocrine components in prostatic carcinomas owing to prognostic and potential therapeutic implications.