To investigate and characterize ERG oncoprotein expression in Japanese patients with prostate cancer (PCa), we evaluated 92 index tumors from Japanese patients who had undergone radical prostatectomy for PCa, using an antibody-based detection method to determine ERG expression. Expression status was compared with clinicopathological findings including patient age, body mass index and preoperative prostate-specific antigen (PSA) levels, specimen Gleason score, pathological stage, positive surgical margin, size of index tumor, prostatic volume, zonal origin of index tumor and biochemical failure. Overall, ERG protein was expressed in 16.3% (15/92) of the index tumors, but not in benign glands. Younger patient age, lower Gleason score and negative surgical margins were found to be independently associated with its expression in the multivariate analysis (P= 0.015, 0.003 and 0.038, respectively). ERG expression was not associated with biochemical failure. Though not statistically significant, ERG expression was more frequently observed in peripheral zone than in transition zone cancers (28.2% vs 10%, respectively). In conclusion, ERG protein was less frequently expressed in this Japanese PCa cohort compared with Western reports. ERG expression was associated with a less aggressive tumor phenotype, and its biological significance needs to be further investigated.