Estrogen-related receptor α (ERRα) was identified as a gene related to estrogen receptor α (ERα) and belongs to a class of nuclear orphan receptors. ERRα binds to estrogen responsive element(s) (ERE) and is considered to be involved in modulation of estrogenic actions. However, biological significance of ERRα remains largely unknown. Therefore, we examined the expression of ERRα in human breast carcinoma tissues using immunohistochemistry (n = 102) and real-time reverse transcription-PCR (n = 30). ERRα immunoreactivity was detected in the nuclei of carcinoma cells in 55% of breast cancers examined, and relative immunoreactivity of ERRα was significantly (P = 0.0041) associated with the mRNA level. Significant associations were detected between ERα and ERE-containing estrogen-responsive genes, such as pS2 (P < 0.0001) and EBAG9/RCAS1 (P = 0.0214), in breast carcinoma tissues. However, no significant association was detected between ERα and pS2 (P = 0.1415) in the ERRα-positive cases (n = 56) or between ERα and EBAG9/RCAS1 (P = 0.8271) in the ERRα-negative group (n = 46). ERRα immunoreactivity was significantly associated with an increased risk of recurrence and adverse clinical outcome by both uni- (P = 0.0097 and P = 0.0053, respectively) and multi- (P = 0.0215 and P = 0.0118, respectively) variate analyses. A similar tendency was also detected in the group of breast cancer patients who received tamoxifen therapy after surgery. Results from our study suggest that ERRα possibly modulates the expression of ERE-containing estrogen-responsive genes, and ERRα immunoreactivity is a potent prognostic factor in human breast carcinoma.