Objective: to estimate the diagnostic and prognostic value of analyzing the abnormal overexpression of the chimeric protein ERG, encoded by the chimeric gene TMPRSS2/ERG, in prostatic neoplasias. Material and methods. A total of 100 prostate adenocarcinoma samples were examined. The presence of tumor and high-grade prostatic intraepithelial neoplasia (hPIN) was verified by immunohistochemical tests using anti-P504S and anti-34βE12 antibodies in serial sections; RT-PCR was employed to analyze the chimeric transcript TMPRSS2/ERG in 30 prostate adenocarcinoma samples. Results. ERG expression was noted in 46% of the adenocarcinomas and in 21% of hPIN. Eight (8%) patients were observed to have heterogeneous ERG expression: the marked reaction in some tumor portions was concurrent with its complete absence in others. Furthermore, there was ERG expression in all cases of intraductal (noninvasive) carcinoma (the foci of intraductal carcinoma were assessed as atypical cribriform lesions by light microscopy). The prognostic value of ERG expression could not be determined at the current stage of the investigation. Conclusion. The relatively low rate of ERG-positive hPIN counts in favor of the limited role of this marker in the differential diagnosis of hPIN. ERG in combination with P504S and 34βE12 is an informative marker for the differential diagnosis of hPIN with intraductal carcinoma.