Tumorous and nontumorous samples from patients with various forms of adult T-cell leukemia/lymphoma (ATLL) were analyzed using the sensitive inverse polymerase chain reaction (PCR) technique. In all samples, oligoclonal expansion of human T-cell leukemia virus (HTLV)-1 bearing T cells were detected, even for the tumorous samples that were mainly monoclonal by Southern blotting. For one case of smouldering ATLL, chemotherapy apparently reduced the number of detectable clones. Taken together with similar data on asymptomatic and symptomatic HTLV-1 carriers without malignancy, it would appear that ATLL appears on a prior background of HTLV-1–initiated oligoclonal expansion. © 1996 by The American Society of Hematology.