Here, we report a rare case of relapsed adult T-cell leukemia-lymphoma (ATL) with evidence of clonal relapse 26 years after initial diagnosis. The patient had been diagnosed with an aggressive form of lymphoma-type ATL 26 years prior and did not receive further ATL treatment for approximately 26 years after achieving complete remission. We used nested PCR to identify the amplification of ATL clone-specific accumulation sites in DNA from hematoxylin and eosin-stained specimens from the patient. Furthermore, the sequence of amplicons obtained from peripheral blood mononuclear cells and lymphoma cells from the previously diagnosed ATL were identical, indicating that a human T-cell leukemia virus-type 1 (HTLV-1)-infected clone identical to the one that recently caused ATL was present in the original lymphoma tissue. Although we were unable to identify this clone as the cause of the previous ATL, the peripheral leukemia cells revealed an ATL clone that was present in the tumor cells of a lymph node diagnosed 26 years earlier. To our knowledge, this is the first report demonstrating survival of HTLV-1-infected clones for a quarter of a century in a patient with recurrent ATL.