Of 1471 sera collected from 1986 to 1989 in Papua New Guinea (PNG), 2.2% were found to be positive for anti-HTLV-1 antibody by successive particle agglutination and immunofluorescence tests. The seropositive rate varied in different provinces and was higher in the coastal areas of the main island and in neighboring small islands than in the highlands of PNG. The frequency of HTLV-1 infection of children was higher, but the age-dependent increase in antibody positivity, generally observed in other HTLV-1 endemic areas of the world, was not clear in PNG. No difference was observed in antibody prevalence in males and females in this study.