We report a case of smoldering adult T-cell leukemia (ATL) with B-cell lymphoma and early gastric cancer. A 64-year-old man was admitted to our hospital because of proteinuria and hypergammaglobulinemia. Systemic lymphadenopathy, "flower cells" in peripheral white blood cells, and hypergammaglobulinemia with monoclonal gammopathy (IgA, lambda type) were found. As Southern blot analysis revealed monoclonal integration of human T-lymphotrophic virus type I proviral DNA in peripheral blood mononuclear cells, he was diagnosed as having smoldering ATL. The tissue specimen of an inguinal lymph node showed proliferation of abnormal lymphocytes which were stained with anti-lambda antibody, indicating B-cell lymphoma. A polypoid lesion in the stomach was histologically diagnosed as early gastric cancer.