Adult T-cell leukemia/lymphoma (ATL) is an aggressive type of leukemia/lymphoma associated with the human T-cell lymphotropic virus (HTLV-I). We describe an adult male patient clinically and pathologically diagnosed as mycosis fungoides and treated with chemotherapy after which complete involution of the lesions occurred. The disease relapsed with confluent dyshidrosis-like vesicles on the palmoplantar regions, followed by disseminated vesiculopapules and associated lymphocytosis. A serological test performed at this time revealed HTLV-I infection, and a diagnosis of chronic ATL was made. Monoclonal integration of HTLV-I was detected in peripheral blood mononuclear cells by inverse long polymerase chain reaction. A skin biopsy revealed spongiosis, Pautrier abscesses, and intraepidermal vesicles with atypical lymphocytes and an infiltration of small and atypical CD4 lymphocytes in the superficial dermis. Proliferative index (Ki-67) was 70%. This is the first reported vesicular cutaneous ATL with confirmation of HTLV-I proviral integration. The delay that occurred in diagnosing ATL was due to the fact that mycosis fungoides and ATL may present the same clinical, histopathological, and immunohistochemical features.