One of the health consequences of the Chernobyl nuclear power plant accident was a radioactive iodine-related increase in the incidence of thyroid cancer in exposed children. This radioisotope is used in the treatment of thyroid cancer and hyperthyroidism patients providing a convenient opportunity to study cytogenetic damage induced by known doses of radioactive iodine in treated patients. We used pancentromeric FISH on micronuclei and chromosome 1 tandem labelling FISH to monitor overall chromosome breakage and loss, 1q12 breakage and decondensation and chromosome 1 numerical abnormalities in buccal cells from 31 radioactive iodine-exposed hyperthyroidism and thyroid cancer patients. The overall outcome of the study, with 250,000 buccal cells analysed, is that there was no radioactive iodine-related increase in the frequency of micronuclei, 1q12 breakage, 1q12 decondensation or chromosome 1 numerical abnormalities. In addition, neither age nor gender, health status nor radioactive iodine dose modulated the frequency of the above cytogenetic end points. Although several uncertainties of these emerging molecular cytogenetic methodologies will require further experimentation, we conclude that, at the reported exposure levels, radioactive iodine did not induce detectable chromosome damage in buccal cells from treated patients.