An analysis of chromosomal aberrations (structural and numerical) and sister chromatid exchange (SCE) was carried out on 16 people exposed to methylmercury through eating fish caught in Cartagena Bay (Columbia), an area of known methylmercury contamination. Fourteen people whose diet consisted mainly of fish caught in another, noncontaminated area of the Atlantic acted as controls. The results showed a significant difference between the experimental and control groups in methylmercury (MM) concentrations measured in hair and peripheral blood. Subsequently, significant differences between levels of organic mercury in blood and hair were found when all the individuals studied were classified in two groups according to their blood mercury levels. When achromatic lesions were included, the frequency of structural chromosome aberrations differed significantly between the exposed and control groups. Furthermore, a significant correlation (p less than 0.05) was found between structural chromosome aberrations and groups (exposed and control). When achromatic lesions were excluded from the analyses, these differences were not found. There was a significant correlation between SCE and age. This is the first report of a study on the frequency of SCE in a population exposed to methylmercury.