Clonal proliferation of human T-lymphotropic virus type I (HTLV-I)-infected cells has been detected by Southern blot analysis and inverse PCR in patients with adult T-cell leukemia, patients with HTLV-I-associated diseases, and even in asymptomatic carriers. Combining inverse PCR with long PCR, we amplified the genomic DNA regions flanking the integration sites of the HTLV-I provirus to detect clones of infected cells. Inverse long PCR revealed that increased virus load was associated with an increase of both the number of cells in each clone and the number of clones. Clonal proliferations were found in both CD4- and CD8-positive cells in a carrier and a patient with HTLV-I-associated neuropathy/tropical spastic paraparesis. These HTLV-I-infected clones persisted over several years in the same carriers, and, moreover, most of the persistent clones were CD4 positive in a HTLV-I carrier. These findings indicate that HTLV-I infection plays an important role in the clonal expansion of lymphocytes and the prolonged survival of CD4-positive cells in vivo. Surviving T-lymphocytes may be susceptible to genetic changes, leading to the onset of leukemia.