Monoclonal integration of DNA sequences related to, but not identical to HTLV-I provirus was detected in the peripheral blood lymphocytes of a Hungarian male suffering from ATL. The patient and his parents showed serological cross-reactivity with both HTLV-I and HTLV-II group-specific antigens. Restriction enzyme analysis with EcoRI, PstI, BamHI, HindIII and SacI revealed structural similarity of the provirus integrated in the DNA of ATL cells to HTLV-I but not to HTLV-II. Data suggest that this provirus and HTLV-I are similar to each other along gag and pol regions, but they are different in the env region.