Androgen metabolism and possible actions are considered to play some roles in human epithelial ovarian neoplasms, but the details have not been well studied. We have examined the expression of 5alpha-reductase type 1 and type 2, which catalyze the conversion of testosterone to more active androgen, 5alpha-dehydrotestosterone, and androgen receptor (AR), using immunohistochemistry (104 cases) and reverse transcription-polymerase chain reaction (RT-PCR) (16 cases) as a first step toward understanding the metabolism and possible actions of androgens in human common epithelial ovarian carcinoma. 5alpha-Reductase type 1 was immunopositive in 75 / 104 cases (72.0%), and 5alpha-reductase type 2 in 52 / 104 cases (50.0%) (P < 0.001). There was no significant correlation between patterns of immunolocalization and clinicopathological parameters examined. Median labeling index (LI) for AR was 17.8% (range 0 - 84.4%) which was significantly higher in serous carcinoma than other histological types (P < 0.001). There was a significant positive correlation between 5alpha-reductase type 1 immunoreactivity and AR LI (P = 0.0027), but no significant correlation was detected in 5alpha-reductase type 2. Results of RT-PCR analysis were also consistent with those of immunohistochemistry. The relatively wide distribution of 5alpha-reductase type 1, and its correlation to AR status in human epithelial ovarian malignancies suggest that this isozyme plays important roles in androgen metabolism and actions in these tumors.