Anti-HTLV III prevalence has been investigated in serum samples of 638 intravenous drug users collected over May 1981 - March 1985 and stored at -20 degrees C. Separately, in a prospective way, we have studied 68 IV drug abusers (53 Genoese and 15 of Sanremo area) of whom we have collected, at least, one serum specimen for each year, starting with 1981. We have also tested for anti-HTLV III presence serum samples of: 91 subjects of Hospital staff (Infectious Diseases Department and Laboratory workers); 32 workers in Therapeutic Communities for drug users and 24 family contacts of anti-HTLV III positive drug users. And then serum samples of two groups of general population collected for other seroepidemiological investigations in 1982 (256 subjects) and 1984 (538 subjects) were tested. No IV drug user was positive in 1981 whilst from 1982 up to 1984 there was a strong rising of the prevalence of anti-HTLV III positive subjects: 2, 22, and 39 per cent, respectively. The prevalence remained about 40% in the first months of 1985. The investigations carried out also show that HTLV III spread in Sanremo area slightly before than in Genoa and neighbourhood. No subject positive for anti-HTLV III has been detected among the Hospital staff and in workers of Therapeutic communities who have more probability to get in contact with infected subjects or their blood, as well as in the general population. A positive case has been discovered in a family contact (the wife of a positive for anti-HTLV III IV drug user). Some epidemiological and public health questions linked to the situation observed, at present, in Liguria, are discussed.