Purpose: In this study, 16 paired samples of colorectal and gastric cancers and adjacent non-neoplastic tissues were analysed for the determination of glutathione S-transferase (GST) activities and the expression of GST-pi. Methods: Western blotting procedure as well as plasma GST-pi levels were used. Results: GST activities were found to be increased in malignant tissues of patient compared with adjacent normal tissues. A significant correlation was detected between GST activity and GST-pi expression in malignant tissues of patients. Plasma GST-pi levels increased in patients compared to aged-matched control subjects. When the patients were grouped according to TNM stage, GST-pi expression in malignant tissues as well as plasma GST-pi levels were higher in patients with more advanced tumor stages. Conclusions: Our results indicate that GST-pi expression in malignant tissues and plasma GST-pi levels in human colorectal and gastric cancers increased depending on the stages of tumor.