A 51-year-old woman was admitted to our hospital complaining of lymphadenopathy in June, 1987. Lymph node biopsy revealed diffuse lymphoma, mixed cell type according to the LSG classification. On hematological examinations, leukocyte has counted 12,400/microliters, of which 15% abnormal lymphocytes containing human T cell lymphotropic virus type-1 (HTLV-1) proviral DNA into the cellular DNA. She was diagnosed as adult T-cell leukemia (ATL). The abnormal lymphocytes in the peripheral blood expressed CD4 but lymph node cells expressed CD8. Thirty months after initial diagnosis, abnormal lymphocytes began to increased and reached 30,500/microliters. Surface phenotype of abnormal lymphocytes changed CD3+ CD4+ CD8- OKla1+ to CD3- CD4- CD8- OKla1-. This change is considered to show the surface phenotypic diversity in ATL cells.