We encountered a patient with anaplastic large cell lymphoma (Ki-1 lymphoma) that originated in the stomach and showed histiocytic lymphoma-like morphology. CD43 antigen was positive, and rearrangement of TCR-beta gene was observed. The lymphoma was the T-cell type. Though no atypical lymphocytes or histological images specific to adult T-cell leukemia were observed, clonal integration of HTLV-1 proviral DNA was noted. Viruses such as HTLV-1 appear to be involved in the development of some anaplastic large cell lymphomas.