A retrospective study on 52,900 individuals tested in 177 Italian Medical Centres for AIDS and related syndromes is reported. The data confirm that I.V. drug users are the group at highest risk in Italy (46% were seropositive and more than one half of these presented PGL or ARC) and that the epidemic probably started in northern Italy and spread south. One third of the examined subjects resulted seropositive for HIV-1 antibodies. ARC was diagnosed in 19% of the symptomatic seropositives. This study is the first national effort to establish a picture of the diffusion of HIV-1 infection in Italy. To reach this aim a periodical survey for the recognition of seropositives should be added to the already existing AIDS notification system.