Background: Adult T-cell leukemia/lymphoma (ATLL) is an aggressive neoplasm of T-lymphocytes associated with human T-lymphotropic virus type I (HTLV-1) infection. As HTLV-1 is endemic in native ethnics in South America, and its infection leads to several chronic diseases as ATLL with poor prognosis, we aimed to present three ATLL cases and to review current literature. Case reports: Two cases were from the mountains of Peru, while one was from an endemic harbor of the country. An acute ATLL patient presented with multipapular infiltration of the skin and died 2 weeks after admission because of septic shock. The two chronic ATLL patients presented with erythematous plaques and erythroderma. They had swollen lymph nodes, lymphocytosis, and atypical lymphocytes on blood smear, with normal biochemical results. They both passed away a few months after diagnosis. Comments: ATLL is developed after years of HTLV-1 carrier status; therefore, physicians should know the principal clinical and laboratory findings in order to make prompt diagnosis. Prognosis is still poor in aggressive and indolent variants, with survival rates from months to a few years. Treatment based on chemotherapy, antiretroviral, and allogeneic stem cell transplantation are improving survival rates but with limited results.