Adult T-cell leukemia/lymphoma (ATLL) is an hematological disease caused by human T-cell lymphotropic virus type 1 (HTLV-1) that develops after 20 years of incubation preferentially when the infection is acquired by vertical transmission. In cases of transmission by transfusion or organ transplant, this time is reduced from 3 months to 3 years. Acute ATLL is difficult to diagnose because it is unusual and has a rapid progression to death. In the Argentine Northwest, where the virus is endemic, ATLL is more frequent, however it is also detected continuously in the rest of the country. The treatment of choice, in the first instance, is the combined use of antivirals. We present a case of acute ATLL developed in a 59-year-old man from Santiago del Estero from which intra-familial transmission of HTLV-1 infection was identified.