Introduction. Epidemiological studies indicate that obesity is associated with an increased risk of 20-25% with several types of cancer.Objective. The frequency of chromosome aberrations was evaluated in lymphocytes frompostmenopausal obese and non-obese women.Materials and methods. Twenty obese and 20 non-obese women, all post-menopause, were recruited.The groups were matched according to age (± 5 years) and place of origin. After signing the consentform, women were interviewed using a structured questionnaire, and a blood sample (5 ml) was drawninto vacutainer tubes. From each sample, lymphocyte cell cultures were established with and without mitomycin C (challenge assay). Afterwards, the frequency of chromosome aberrations were recordedfor each group and treatment. Data were analyzed using the statistical program SPSS, v. 14.0.Results. Obese women had a higher frequency of chromosome aberrations when compared with nonobesewomen. After exposing the cell cultures to mitomycin C, obese women presented an increasein the number of total chromosome aberrations in comparison to non-obese women (3.7± 0.6 vs.2.70±0.6; p=0.001).Conclusions. The higher frequency of chromosome aberrations in lymphocytes from postmenopausalobese women compared to non-obese women suggested differences in the DNA repair capacity. Thismay indicate an association between genomic instability and the higher incidence of cancer in thispopulation.