Estrogens have an important role in the development and progression of breast cancer. 17beta-Hydroxysteroid dehydrogenase type 1 (17HSD1), type 2 (17HSD2), and type 5 (17HSD5) are associated with sex steroid metabolism in normal and cancerous breast tissue. The mRNA expressions of the 17HSD1, 17HSD2, and 17HSD5 enzymes were analyzed in 794 breast carcinoma specimens by using tissue microarrays and normal histologic sections. The results were correlated with the estrogen receptor alpha (ER-alpha) and beta (ER-beta), progesterone receptor, Ki67, and c-erbB-2 expressions analyzed by immunohistochemical techniques and with the Tumor-Node-Metastasis classification, tumor grade, disease-free interval, and survival of the patients. Signals for 17HSD1 mRNA were detected in 16%, 17HSD2 in 25%, and 17HSD5 in 65% of the breast cancer specimens. No association between the 17HSD1, 17HSD2, and 17HSD5 expressions was detected. A significant association was observed between ER-alpha and ER-beta (P = 0.02; odds ratio, 1.96) expressions. There was also a significant inverse association between ER-alpha and 17HSD1 (P = 0.04; odds ratio, 0.53), as well as ER-alpha and 17HSD5 (P = 0.001; odds ratio, 0.35). Patients with tumors expressing 17HSD1 mRNA or protein had significantly shorter overall and disease-free survival than the other patients (P = 0.0010 and 0.0134, log rank). The expression of 17HSD5 was significantly higher in breast tumor specimens than in normal tissue (P = 0.033; odds ratio, 5.56). The group with 17HSD5 overexpression had a worse prognosis than the other patients (P = 0.0146). ER-alpha also associated with survival (P = 0.045). Cox multivariate analyses showed that 17HSD1 mRNA, tumor size, and ER-alpha had independent prognostic significance.