Glutathione S-transferase (GST) is known to play a key role in the detoxification and reduction of reactive oxygen species (ROS). Thus, we assessed GST activity and GST-pi expression in relation to oxidative stress and apoptosis in breast cancer. Tumor tissues from 32 breast cancer patients were evaluated for GST activity and thiobarbituric acid reactive substances (TBARS) that are by-products of oxidative stress. Four-micron sections of formalin-fixed, paraffin embedded tumors were stained immunohistochemically with anti-GST-pi. Apoptotic cells were detected by in situ end labeling of DNA fragments using a commercial kit. TBARS levels were significantly higher in breast cancers of older patients. GST-pi expression was up-regulated in breast cancers that exhibited higher oxidative stress and associated with higher GST activity. Apoptosis in GST-pi negative tumors was not correlated with GST activity, but GST-pi positive tumors within the same range of oxidative stress showed a reduction in apoptosis as well as an increased GST activity. This correlation was absent in GST-pi positive tumors experiencing higher oxidative stress. GST-pi expression may influence the level of GST activity and delay apoptosis in breast cancer. However, GST-pi expression in tumors with higher levels of oxidative stress may not be sufficient to abrogate the deleterious effects of ROS.