Adult T-cell leukemia/lymphoma (ATLL) is a malignant neoplasia of mature CD4+ T lymphocytes,resulting from infection with human T-lymphotropic virus type 1 (HTLV-1), with several systemic and cutaneous manifestations. We present two cases of adult T-cell leukemia/lymphoma, in patients from the Colombian Southwestern region, whose diagnoses were confirmed by histology, immunohistochemistry, flow cytometry, ELISA and Western blot tests. We also discuss about the virus and how to make this diagnosis in countries like Colombia.