Glutathione S-transferase pi (GST-pi) is a phase II detoxification enzyme whose expression is increased in estrogen receptor (ER)-poor breast cancers and in breast cancers resistant to certain chemotherapeutic agents. The aim of this study was to investigate the immunohistochemical expression of GST-pi in invasive breast carcinoma and to correlate the findings with those of nuclear morphometry. Formalin-fixed paraffin-embedded tissue specimens obtained from 21 invasive breast cancers and 16 adjacent (benign) tissues were immunohistochemically stained using polyclonal anti-human GST-pi antibody. There was positive (defined as >10% immunoreactive tumor cells) but variable expression of GST-pi in 10 (48%) cases. Nuclear morphometry in these 10 tumors revealed immunoreactive malignant cells to be larger (mean area 41.7+/-1.0 microm2) and more rounded in form when compared with non-staining cancer cells (mean area 28.7+/-0.7 microm2). It was also observed that GST-pi immunonegative tumor cells in GST-pi expressing tumors had different morphologies from malignant cells in the remaining 11 (52%) cancers that were regarded as GST-pi negative. Increased GST-pi expression determined by the percentage of positively staining tumor cells, was found to be significantly correlated with increased variability in nuclear area and perimeter (Spearman's rho=0.821, p=0.044 for both) in the subset of node-positive tumors. Our findings suggest that there exists two sub-populations of cancer cells with distinct nuclear morphologies in GST-pi positive tumors; factors other than GST-pi expression are likely to have a phenotypic effect on breast cancer cells; and there may be a special significance of this enzyme in axillary node-positive breast tumors.