We report the first complete nucleotide sequence of an adult T cell leukaemia virus/human T cell leukaemia virus type I (ATLV/HTLV-I) isolate from a British patient of Caribbean origin. Sequence comparisons of our proviral clone (HS-35) with other molecular clones are shown. We note the strong sequence conservation between isolates of Caribbean and Japanese origin (2.3% divergence), but demonstrate the higher homologies existing between isolates originating from similar geographical areas (approximately 1% divergence). Implications for the origin, evolution and dissemination of the ATLV/HTLV-I subgroup are discussed. Analysis of defective proviral clones isolated from the same genomic library is also reported, and suggests a pattern of proviral sequence deletions during the biogenesis of defective proviruses.