After the Chernobyl nuclear accident, a dramatic 131I-related increase in the incidence of thyroid cancer has been reported in exposed children. However, little is known about the eventual genotoxic effects of 131I in exposed humans. Thyroid cancer patients are usually treated with 131I and, therefore, they provide us with an opportunity to study cytogenetic damage induced by known doses of this radionuclide. FISH techniques have been employed to study the origin of micronuclei as well as X chromosome nondisjunction and X chromosome numerical abnormalities in lymphocytes from 131I-treated women suffering from thyroid cancer. Blood was sampled before and 1 week after 131I treatment. Cells were analysed with either pancentromeric FISH to classify micronuclei or X chromosome centromere-specific FISH in mononucleated and binucleated cells to evaluate X chromosome numerical abnormalities and non-disjunction respectively. Our data indicate that 131I-induced clastogenic and age-dependent aneugenic effects in the lymphocytes of exposed patients. The X chromosome was not preferentially involved in the aneugenic effect induced by 131I. It is concluded that besides its major clastogenic effect 131I can also induce an X chromosome-independent aneugenic activity mainly in patients with spontaneous proneness to chromosome loss.