The activities of total superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (CAT) enzymes were measured in cancerous and non-cancerous adjacent tissues from 15 patients with follicular thyroid cancer containing single nodule. SOD and GSH-Px activities were found lower but malondialdehyde levels higher in cancerous tissues compared with those of noncancerous ones. However, no difference was found between CAT activities of the tissues. Activity decrease of GSH-Px enzyme in cancerous tissue was greater than that of SOD enzyme. Results suggested that enzymatic free radical defense system was significantly impaired and lipid peroxidation increased in the cancerous human thyroid tissues.