Subjects for the study were heterosexual IV drug users enrolled in five major opiate addiction treatment programs, and a sample of addicts who were not undergoing treatment, from December 1984 to October 1985. Each subject was interviewed with a standard questionnaire to obtain demographic, sexual, drug use, and behavioral information. Results indicate a 10-percent prevalence of antibody to the HIV among 281 heterosexual IV drug users, suggesting an emerging epidemic of AIDS among drug users in San Francisco. Also found was an increased risk of seropositivity with increasing numbers of persons with whom needles are regularly shared. HIV infection in San Francisco's IV drug users is significantly more prevalent in blacks and Latinos than in whites. HIV infection in San Francisco drug users lags behind homosexual men by 4 years because there appears to be little overlap between homosexual and heterosexual drug-using populations in San Francisco, and addicts' social isolation and lack of shooting galleries in San Francisco may have provided a protective barrier to early introduction of HIV to drug users. The most effective strategy to prevent HIV infection in IV drug users is to eliminate IV drug injection altogether. 3 tables and 19 references.