The prevalence of anti-LAV/HTLV III antibodies in serum samples of individuals at high risk for AIDS and of healthy individuals living in the province of Catania is reported. Serum samples were collected over the period 1983-86 from 220 drug addicts adults, 339 from multitransfused thalassemia patients, 71 from hemophiliacs, 22 from subjects of the hospital staff who had had contacts with positive subjects, 9 from family contacts of positive multitransfused thalassemics and 7270 from blood donors. Prevalence of antibodies among drug addicts was 14% and a positive subject was found as early as 1983. This result suggested that LAV/HTLV III was already present in Catania at that year. Among the thalassemics and the hemophiliacs the prevalence was 2.3% and 21% respectively. No positive subject was found among hospital staff nor among family contacts of positive thalassemic subjects. Three positive cases have been found among the blood donors and even though these are drug addicts, it is important to underline the necessity of a regular screening of the blood units to prevent LAV/HTLV III diffusion.