Two cases of adult T-cell leukemia (ATL) associated with gastric cancer and lung cancer were reported. The first case was a 73-year-old man, who was diagnosed as ATL, because of the presence of OKT3⁺, 4⁺, 11⁺, Tac⁺ atypical lymphocytes, lack of antigenic modulation of Tac antigen and expression of human T-cell leukemia virus associated antigens in short-term culture. He was induced in complete remission status by combination chemotherapy with CVP (cyclophosphamide (CY), Vincristine (VCR), and prednisolone (PSL)), then subtotal gastrectomy was carried out successfully. However, ATL recurred after the operation and he died of pseudomenbranous colitis under combination chemotherapy with CVP. Second case was a 53-year-old man, who was also diagnosed as ATL with similar features of the first cases. Incidentally he was found the lung cancer in right lobe. He was induced in partial remission status by the administration of PSL and alpha-interferon, then lobectomy of middle and lower lobes of right lung was carried out. His post-operative course was satisfactory by the administration of alphainterferon (provided by Sumitomo Pharmaceuticals), and he was discharged five months after the operation. In our clinic, double cancer had been found in five of sixteen patients with ATL. The incidence of double cancer (31.3%) is higher than that of other hematological disorders. Possible association of HTLV-I or the defect of the host immune surveillance mechanisms with double cancer were suggested.