Down's syndrome (DS) has the highest incidence among chromosomal disorders and is a predisposing factor in acute leukemia pathogenesis. DS patients are sensitive to both physical and chemical inducers at the DNA level. Studies on β-carotene, an antioxidant, suggested that there is a relationship between high β-carotene diet and reduced tumor incidence in humans indicating that β-carotene is a chemopreventive agent against cancer. Sister chromatid exchange (SCE) is known as a sensitive parameter among the genotoxicity tests. In this study, we aimed to investigate the in vitro effect of β-carotene on SCE frequencies in 7 DS patiens and 7 healthy controls aged between 0-16 years. A direct leukomogenic agent Mitomycin-C (MMC) was used as a powerful SCE inducer. Addition of MMC to the cultures alone resulted in a significant enhancement of SCE frequencies in both groups when compared to the spontaneous values. In the study, β-carotene seemed to decrease MMC induced mean SCE/cell values, but did not have an effect on unstimulated cells. As this is a limited study, it is hard to conclude that β-carotene is a chemopreventive agent in DS patients, although our results seem to support other investigators' reports.