Although the intra-tumoral estradiol (E2) level is thought to be more closely related to the E2-inducible events such as progesterone receptor (PR) induction and tumor growth than the serum E2 level, prognostic significance of the intra-tumoral E2 level in breast cancer patients has yet to be studied. E2 level in tumor tissues obtained from 90 primary breast cancer patients were measured by the high-pressure liquid chromatography-radioimmunoassay method. Relationship between the intra-tumoral E2 level and the various clinicopathological factors including the prognosis was investigated. E2-high tumors (n=45) were significantly more likely to be estrogen receptor (ER)-positive tumors and PR-positive tumors than E2-low tumors (n=45). A significant correlation was observed between the intra-tumoral E2 level and PR level by a simple curve fit test (r=0.39, P<0.01) in the ER-positive tumors. Univariate analysis showed that lymph node status, intra-tumoral E2 level, ER, and PR were significant prognostic factors, and multivariate analysis showed that only lymph node status and PR status were significant independent prognostic factors. PR is induced by E2 through ER in human breast cancers. PR, which warrants both the presence of functional ER and the enough level of intra-tumoral E2, can serve as a better prognostic factor than intra-tumoral E2 level or ER in primary breast cancer patients probably because PR is a better marker of estrogen-dependency of tumor growth than intra-tumoral E2 level or ER.