Estrogen-related receptor (ERR) is a nuclear receptor that modulates the estrogen-signaling pathway. Here, we investigated the expression of both ERRbeta and ERRgamma in human prostate tissues. Using original rabbit polyclonal anti-ERRbeta and anti-ERRgamma antibodies, the expression of ERRbeta and ERRgamma was evaluated by immunohistochemical analysis of cancerous lesions (n = 107) and benign foci (n = 92), obtained by radical prostatectomy. Stained slides were evaluated for the proportion of immunoreactive cells and their staining intensity. Total immunoreactivity scores (IR scores; range, 0-8) were calculated as the sum of the proportion and intensity scores. The relationship between the clinicopathological characteristics of the patients and the expression of the three ERRs (ERRalpha, ERR beta, and ERR gamma) was evaluated. IR scores for ERRbeta and ERRgamma were significantly lower in cancerous lesions than that in benign foci (P < 0.0001, for both). Clinicopathological analyses revealed that the patients with low ERRgamma IR scores (<or=4) tended to show poor cancer-specific survival (P = 0.07). Then, we used data from our previous study (Fujimura T., Int J Cancer, 2007; 120: 2325-30). Patients with a high IR score for ERRalpha and a low score for ERRgamma showed significantly poorer cancer-specific survival than those with a low IR score for ERRalpha and a high score for ERRgamma (P = 0.0003). We demonstrated the differential expression of ERRbeta and ERRgamma in prostate tissue. The combined evaluation of the expression of ERRalpha and ERRgamma could be a significant prognostic factor for prostate cancer.