The drug addiction unit of USSL 25 in the Upper Seriana Valley, Clusone (BG), have dealt with HIV infection since 1985. Over the past four years 357 subjects, 90 (48.1%) of whom proved to be HIV-positive using ELISA methods and Western control blotting. Five drug addicts included in the study underwent seroconversion. Clinical symptoms evolved in thirteen-drug addicts and five were diagnosed with AIDS. Two children of seropositive mothers were found to be HIV-positive and therefore form part of the high-risk non-drug-addict population. All subjects who were not at risk were found to be seronegative. The ned for primary preventive therapy is underlined.