The prevalence of human immunodeficiency virus (HIV) antibodies and the symptoms induced [persistent generalized lymphadenopathy (PGL), AIDS-related complex (ARC), acquired immunodeficiency syndrome (AIDS)] was evaluated in several groups of intravenous (IV) drug abusers in three large Italian cities (Milan, Bologna, and Rome). The earliest evidence of seropositivity in sera collected from patients with acute viral hepatitis dates back to 1979 in Milan and to 1981 in Bologna with peaks in 1983 in both cities. In two groups of IV drug addicts on methadone-maintenance treatment at assistance centers, the prevalence of seropositivity differed sharply between Rome (33.3%) and Milan (69.3%) in 1985. Rates of seroconversion were also found to be higher in Milan than in Bologna and Rome. When a population of IV drug abusers voluntarily attending centers for surveillance of AIDS and/or ARC were investigated, high levels (range 55.2-81.5%) of seropositivity were found in the three cities. ARC prevalence among seropositives was very high (range 48.144.2% in 1985) in the three cities. The evolution rate to AIDS in Milan was higher among those attending a center for AIDS surveillance (7.4%) than among those attending an assistance center for methadone treatment (0.9%). These data are compatible with the hypothesis that virus infection among IV drug abusers originated in and then spread widely in Northern Italy (Milan first and then Bologna). Both the first appearance and subsequent spread of virus infection are in keeping with the reported occurrence of AIDS cases in the corresponding three regions of Milan, Bologna, and Rome.