Erythrocyte antioxidant glutathione and malondialdehyde levels in erythrocytes and plasma glutathione S-transferase levels were estimated in patients with colorectal cancer and compared to controls. Further, the patients underwent four weeks of radiotherapy with adjuvant chemotherapy. The same parameters were estimated after four weeks of radiotherapy and compared.with pretreatment levels. It was observed that there was a decrease in erythrocyte glutathione and malondialdehyde levels in patients with colorectal cancer compared to controls, but not in case of GST. However, after chemoradiotherapy, there were no statistically significant differences in all the parameters studied.