BACKGROUND Glutathione S-transferase is often up-regulated in neoplastic tissues. A single previous study found a loss of expression associated with carcinogenesis of the prostate. METHODS To extend these results, the authors performed immunohistochemical staining for the pi-class of glutathione S-transferase (GSTpi) on 74 archival sequential prostate specimens. The antibody used was derived from rabbits immunized against purified human GSTpi. Paraffin blocks containing both benign tissue and adenocarcinoma were studied. RESULTS Heterogeneous expression of GSTpi in benign acini was found in 96% of cases, but GSTpi was not expressed in 95% of invasive adenocarcinomas of the prostate, nor was it expressed in any of the foci of high grade prostatic intraepithelial neoplasia. Basal cells of benign acini showed strong, diffuse staining for GSTpi, whereas the secretory luminal epithelium expressed GSTpi weakly and focally. CONCLUSIONS This study confirms the down-regulation of GSTpi in adenocarcinoma of the prostate and shows that the loss of GSTpi expression is a phenotype associated with malignant transformation.