Androgen, acting via the androgen receptor (AR), is associated with the development and progression of prostate cancer. Anti-androgen therapy is widely used to manage prostate cancer. However, the conversion of the tumor from a hormone-sensitive to a hormone-insensitive status causes such therapy to fail. Several mechanisms have now been put forward for this conversion, including neuroendocrine (NE) differentiation of the tumor cells. In this study, we evaluated the prognostic significance of tumor-cell proliferation activity, NE differentiation and AR expression. Formalin-fixed, paraffin-embedded sections were prepared from 42 patients with adenocarcinoma of the prostate. Using antibodies to AR, the Ki-67 antigen (MIB-1), chromogranin A and synaptophysin, immunohistochemical expression of AR, tumor proliferation activity and NE differentiation were analyzed. Our study revealed that AR expression was significantly lower in adenocarcinoma (52.2 +/- 27.1%) than in non-tumorous prostate tissue (68.3 +/- 18.3%; P < 0.001). NE differentiation was found in 50% of the tumors, which was correlated with the Gleason score (P < 0.05). An univariate analysis revealed a significant correlation between progression-free survival with both AR expression (P < 0.01) and proliferation activity (P < 0.001). NE differentiation was not a prognostic factor in this study.