Mycosis fungoides/Sézary syndrome(MF/SS) is considered as a distinct clinical entity in the latest classifications, including the revised European-American classification of lymphoid neoplasm(REAL), and the ones by the WHO and the European Organization for Research and Treatment of Cancer(EORTC). In Japan, where is an endemic area for human T-cell lymphotropic virus type 1(HTLV-1), we have been facing some problems concerning the differential diagnosis of MF/SS. We have observed many cutaneous types of adult T-cell leukemia/lymphoma(cATL/L) that can initiate as erythematous infiltrating plaques similar to MF or as an erythrodermic lesion similar to SS and also as tumoral and nodular lesions, solitary or multiple that resemble d'emblée type MF. The importance of the differential diagnosis between MF/SS and cATL/L lies on the significant difference on the prognosis. It is worth to differentiate MF/SS from cATL/L, especially in MF patients positive for HTLV-1 antibody. We emphasize on the characteristics that differentiate MF/SS from cATL/L as well as on the new findings regarding clinical, histopathological, phenotypical and genotypical aspects of MF/SS.