BACKGROUND Adult T-cell leukemia/lymphoma (ATLL) is a distinctive peripheral T- lymphocytic malignancy associated with human T-cell lymphotropic virus type 1 (HTLV-1). It may closely resemble other skin lymphomas, particularly mycosis fungoides (MF). METHODS A 38-year-old woman presented some ellipsoid scaling patches lasting 18 months and developed a large tumoral lesion in the abdomen, which were previously diagnosed as MF. Although histopathologic and immunohistochemistry findings were in consonance with this diagnosis, the fast progression of the disease raised the suspicion that it could represent another type of T-cell lymphoma. The work-up revealed a positive anti-HTLV-1 serology and molecular studies confirmed the monoclonal integration of HTLV-1 provirus into neoplastic cells of the skin, but not into circulating lymphocytes. Extensive investigations were unable to demonstrate any systemic involvement. The final diagnosis was of primary cutaneous type of ATLL. The patient was submitted to a chemotherapy regimen with cyclophosphamide, doxorubicin, vincristine and prednisone, later to conjugated dexamethasone and surgical cytoreduction and then to a second line treatment with gemcitabine, resulting in partial response. A bone marrow heterologous transplantation was performed, but failed to achieve a sustained remission. CONCLUSIONS ATLL is a rare lymphoid malignancy in non-endemic HTLV-1 areas, the diagnosis of which could be missed if not highly suspected. In addition to the four subtypes of Shimoyama classification (acute, lymphomatous, chronic and smoldering), a fifth one denominated primary cutaneous and characterized by presence of lesions only in the skin had been proposed and is herein exemplified.