In this study we examined the immunohistochemical localization of sex steroid receptors for estrogen alpha (ER alpha) and ER beta, progesterone-A (PR-A) and PR-B, and androgen (AR) in human thymoma (n = 132) and correlated these findings with various clinicopathological parameters. We used RT-PCR and real-time PCR to further study the expression of these receptors in 20 thymoma cases. Immunoreactivity for all sex steroid receptors was detected in the nuclei of thymoma epithelial cells. The percentage of immunopositive cases and the H-score values for each receptor (mean +/- SD) were: ER alpha, 66% and 85.8 +/- 80.2; ER beta, 7% and 7.2 +/- 8.7; PR-A, 4% and 2.7 +/- 4.9; PR-B, 49% and 55.8 +/- 68.3; and AR, 15% and 14.1 +/- 11.7, respectively. The results of real-time PCR were consistent with those of immunohistochemistry, especially results for ER alpha, PR-B, and AR. A significant positive correlation was detected between immunoreactivity for ER alpha and PR-B. ER alpha immunoreactivity was inversely correlated with tumor size, clinical stage, WHO classification, and Ki-67 labeling index. In addition, the status of ER alpha immunoreactivity was significantly associated with a better clinical outcome in thymoma patients. Results from our study suggest that estrogens may inhibit thymoma growth via ER alpha, and that ER alpha immunoreactivity may act as a prognostic factor in human thymoma.