An electron microscopic study of adult T-cell leukemia, which might be a new distinct disease entity of T-cell malignancies, is presented. The most characteristic feature of adult T-cell leukemia was an extremely irregular nucleus with condensed heterochromatin in the nuclear periphery, the greater part of which was very similar to the small-cell variant form of Sézary syndrome, although some nuclei were similar to normal-appearing lymphocytes or atypical lymphoid cells with slightly indented nuclei or Sézary cells. Nuclear pockets containing tubuloreticular structures and filamentous structures etc., were also characteristic. Regarding the cytoplasmic features, the appearance of clustered dense bodies seemed to be a distinguishing feature of adult T-cell leukemia. Tubuloreticular structures were also observed in some cases. The nature of the test tube-shaped or ring-shaped forms in rough endoplasmic reticulum, which appeared in one case, is not clear.