This study was designed to evaluate the influence of genetic markers on the seropositivity of offspring of HTLV-I positive mothers in Tumaco, Colombia, an endemic area for HTLV-I infection and a site where there exists a racially mixed population of Black and Caucasian ancestors. 33 HTLV-I seropositive women with at least one offspring were studied. A total of 111 offspring were tested using hemaglutination-inhibition for testing sera for the allotypic markers G1m (1, 2, 3, 17) and G3m (5, 6, 13, 21). Potential risk factors such as mother s age at child's birth, mother's age at the time of the study, breastfeeding months, TSP vs. asymptomatic HTLV-I carrier, sibship's size, children's age and sex, were not found to be associated with mother to child transmission. Mother's Negroid genetic marker genotype (1, 17, 5, 13/1, 17, 5, +/- 13) was marginally associated with mother to child transmission of HTLV-I (P=0.0057; OR=11.97; CI=0.92-155.96)