To determine the genotoxic risk associated with diagnostic exposure to low doses of iodine 131 (I-131), sister chromatid exchange (SCE) analysis was performed in lymphocytes of 18 non-smoking women who received 370 kBq (10 mu Ci) intravenous I-131 sodium iodide as an adjuvant for scintigraphy for diagnosing thyroid nodularity. SCE frequencies were measured before and after I-131 administration. SCE results in the pre-treated phase were regarded as control. Although SCE values 24 h after I-131 administration did not show a significant increment (p > 0.05), there was a significant increase 72 h after treatment (p < 0.05). These results indicate that genetic damage might be induced by low dose of I-131. (C) 1997 Elsevier Science B.V.