Post-operative radioiodine treatment of differentiated thyroid cancer occupies a well determined place in the treatment policy of this disease. Since little is known about erythrocytes oxidant and antioxidant status in patients with thyroid cancers after radioiodine, treatment, we measured erythrocyte malondialdehyde (MDA) levels, as a marker of lipid peroxidation, erythrocyte reduced glutathione (GSH) levels and activities of GSH-Peroxidase and GSH-Reductase as antioxidants. We found that erythrocyte MDA levels were significantly higher, and erythrocyte GSH levels and activities of GSH-related enzymes were significantly lower in thyroidectomised patients after surgery than in healthy controls. Additionally, according to their thyroid hormone levels the patients had hypothyroidism at this time. In patients 2 days after radioiodine treatment both MDA and GSH levels and GSH-related enzyme activities were significantly increased when compared to their own initial levels. In conclusion, we, suggested that radioiodine could facilitate or induce significant oxidant/antioxidant changes in erythrocytes-indirectly as a result of the internal radiotherapy with radioiodine. (C) 1998 The Italian Pharmacological Society.