The genotoxicity of fluoride in vivo in seven patients with osteoporosis was cytogenetically investigated. The patients were treated with fluoride-containing formulations (disodium monofluorophosphate and sodium fluoride) for a period of 15 months up to 49 months. Fluoride intake ranged from 22.6-33.9 mg F-/day and serum fluoride concentrations were between 0.1 mg F-/l and 0.2 mg F-/l. Peripheral blood lymphocytes of these patients were cultured in vitro and examined for chromosomal aberrations, micronuclei in cytokinesis-blocked binucleated lymphocytes as well as cell cycle progression. When a comparison was made between patients' group and a matched control group, it was found that fluoride at the tested concentrations had no detectable genotoxic potential in human lymphocytes in vivo.