The prevalence of infection with human T‐cell leukaemia virus (HTLV‐I) was studied in Madang Province on the north coast of Papua New Guinea. Serum specimens collected from non‐pregnant women in 17 villages were tested for anti‐HTLV‐I by gelatin particle agglutination screening and confirmed by immunofluorescence and Western blotting. Overall, 13.9% of subjects were antibody‐positive, with the prevalence of antibodies varying from <10% to 30% in villages situated less than 10 km apart. Two groups of migrant women were identified, and in both a parity‐related increase in antibody prevalence which occurred only after marriage suggested that the predominant mode of transmission in migrant women was sexual. There was no parity‐associated increase in anti‐HTLV‐l in indigenous women, and in contrast to migrant women, nulliparous indigenous women had a high prevalence of antibody (16.8% vs. 0%; p = 0.005). Vertical transmission cannot be excluded in indigenous women. No correlation was detected between the prevalence of anti‐HTLV‐l and a variety of indices of malarial infection.