Estrogen sulfotransferase (EST; SULT 1E1 or STE gene) sulfonates estrogens to inactive estrogen sulfates, whereas steroid sulfatase (STS) hydrolyzes estrone sulfate to estrone. Both EST and STS have been suggested to play important roles in regulating the in situ production of estrogens in human breast carcinoma tissues. However, the expression of EST has not been examined in breast carcinoma tissues, and the biological significance of EST and STS remains unknown. Therefore, in this study, we examined the expression of EST and STS in 35 specimens of human breast carcinoma tissues using immunohistochemistry, reverse transcription-PCR (RT-PCR), and enzymatic assay. EST and STS immunoreactivity was also correlated with various clinicopathological parameters, including prognosis to examine the biological significance of these enzymes in 113 breast carcinomas. EST and STS immunoreactivity was detected in carcinoma cells and significantly associated with their mRNA levels (P = 0.0027 and 0.0158, respectively), as measured by RT/real-time PCR, and enzymatic activities (P = 0.0005 and 0.0089, respectively) in 35 breast carcinomas. In breast cancer tissues examined by laser capture microdissection/RT-PCR analyses, the mRNA for EST was localized in both carcinoma and intratumoral stromal cells, whereas that of STS was detected only in carcinoma cells. Of the 113 invasive ductal carcinomas examined in this study, EST and STS immunoreactivity was detected in 50 and 84 cases (44.2 and 74.3%), respectively. In these cases, EST immunoreactivity was inversely correlated with tumor size (P = 0.003) or lymph node status (P = 0.0027). In contrast, STS immunoreactivity was significantly correlated with tumor size (P = 0.0047). Moreover, EST immunoreactivity was significantly associated with a decreased risk of recurrence or improved prognosis by both uni (P = 0.0044, and 0.0026, respectively) and multivariate (P = 0.0429 and 0.0149, respectively) analyses. STS immunoreactivity, however, was significantly associated with an increased risk of recurrence (P = 0.0118) and worsened prognosis (P = 0.0325) by univariate analysis. Results from our present study suggest that immunoreactivities for both EST and STS are associated with their mRNA level and enzymatic activity and that EST immunoreactivity is considered to be a potent prognostic factor in human breast carcinoma.