Human T lymphotropic virus I (HTLV-I) is a retrovirus associated with adult T cell leukemia/lymphoma, tropical spastic paraparesis and possibly with other diseases. The actual incidence of HTLV-I carriers in Asia and Africa is difficult to assess and values of 0–37% have been reported. A study in Papua New Guinea by Yamaguchi et al. [6]found 1.8% positive for antibody to HTLV-I and this area is geographically close to the eastern part of Indonesia. A collaborative study was carried out to establish the seroprevalence of HTLV-I in Indonesia using the same method as that used standardly by Yamaguchi et al. in Japan. In a first step we measured antibody to HTLV-I in 657 subjects including 127 healthy persons around Jakarta, 451 patients with various nonhematological diseases and 79 blood donors. The majority of these healthy persons and patients came from various tribes of west Indonesia. All samples were initially screened for HTLV-I by the aggutination test. Serum that caused agglutination of the final dilution of 1:16 or more and/or showed positivity on ELISA was subjected to immunofluorescence testing using HTLV-I-producing MT-2 cells and Western blot analysis. Of the 657 samples, only 1 sample (0.15%) was found to be positive by the agglutination test, but this sample was negative when subjected to WB. From these result we concluded that there was very low seroprevalence of HTLV-I in the Jakarta area, which was also reflected in the western part of Indonesia. Routine examination of blood donors for HTLV-I in this area may not be necessary.