Human T-cell lymphotropic virus type I (HTLV-I) is associated with adult T-cell leukaemia/lymphoma (ATLL) and tropical spastic paraparesis (TSP/HAM) in endemic and non-endemic areas. Serological studies have shown that HTLV-I is prevalent in some Latin American countries such as Brasil, Chile, Colombia, Perú and Uruguay. We describe here the clinical and laboratory features of five cases of ATLL diagnosed in Argentina. All patients (4 males, 1 female; median age 48.2 years) were of caucasian origin; 4 born in Argentina and 1 in Chile. High risk factors for HTLV-I infection were not apparent in Argentina patients, whereas the Chilean resident, who was a promiscuous heterosexual, travelled through Chile frequently. Positive results for antibodies to HTLV-I were detected in all five cases and in some of their relatives. This report suggests that HTLV-I infection may be endemic in, Argentina where TSP has also been described.