We describe 4 cases of adult T-cell leukemia (ATL) with unusual morphology and aberrant immunophenotype. All patients were Japanese and born in the Nagasaki district, an area endemic for HTLV-I. Peripheral blood and/or bone marrow films revealed bizarre giant cells with and without large nucleoli; the cells were 5 to 6 times the diameter of erythrocytes, resembling Hodgkin's cells. Some peripheral blood cells were morphologically similar to prototypic ATL cells, while many other cells in the bone marrow showed unusual morphology. Furthermore, leukemic cells had aberrant immunophenotypes such as the CD8-positive type in patients 1 and 2, the CD4-.CD8- double-negative type in patient 3, and the CD5 antigen defect in patient 4. All patients had marked elevations of the serum calcium and LDH and organomegaly, while all had a short survival. Anti-HTLV-I antibodies and provirus DNA monoclonality were demonstrated in all patients. The results suggested that the unusual morphology and aberrant ATL cell immunophenotype may be indicative of a high grade malignant behaviour of ATL.