Serum samples of 769 healthy Venezuelan donors were assayed for natural antibodies to HTLV-I by the ELISA technique. Specific HTLV-I antibody prevalence was 6.8% but varied from 1% in Caracas to 13.7% in the Amazonas region and the State of Zulia. Adults infected with Trypanosoma cruzi had the highest HTLV-I antibody prevalence of 15%. Areas of high antibody prevalence were correlated most strongly with the presence of arthropod-borne diseases and to a lesser extent with socio-economic factors. Genetic factors were not correlated with antibody prevalence. Antibodies were seen in children as young as 3 years of age in the most endemic areas. Antibody titers increased with age, suggesting continuous exposure to the virus. The data provide clues for elucidation of the geographic variation in HTLV-I antibody prevalence seen Venezuela and other HTLV-I endemic areas. In addition, they further confirm the Caribbean region as being endemic for HTLV-I and extend this region to inland areas of South America.