PUVA therapy is known to be effective for treating cutaneous T-cell lymphoma (CTCL). Considering that adult T-cell leukemia/lymphoma (ATL) represents a T-lymphocytic proliferative disorder similar to CTCL, it is likely that PUVA therapy is effective for treating ATL. We applied PUVA therapy to a 53 year old man with acute (i.e. crisis)-type ATL associated with generalized erythematous papules, and succeeded in achieving a complete remission (CR). To elucidate the mechanism of PUVA therapy in this case, we compared ultrastructures of leukemic cells obtained before and after PUVA therapy by electron microscopy, and found that PUVA therapy caused apoptosis in leukemic cells in the peripheral blood. In this paper, the antitumor mechanism of PUVA is discussed, and its efficacy is emphasized.