Assuming that the clonal expansion of T cells harbouring the human T-cell leukemia virus type 1 (HTLV-1) provirus is a central feature of HTLV-1 infection, the identification of such cells was sought among a series of 19 asymptomatic carriers and 19 cases of tropical spastic paraparesis/HTLV-1 associated myelopathy (TSP/HAM) devoid of malignancy. Two PCR based protocols designed to amplify the host cell-HTLV-1 proviral integration sites were used. In all cases large numbers of proliferating clones could be identified. The proportion of some clones was> 1/1500 peripheral blood mononuclear cells (PBMCs) with the suggestion that their number increased as a function of age among asymptomatic carriers.