The study population consisted of IV drug abuser patients with AIDS or ARC hospitalized from July 1981 to March 1983 in the Bronx, New York. A comparison control group of IV drug abusers with no evidence of AIDS or ARC was taken from hospitalized IV drug abusers and healthy IV drug abusers enrolled in a treatment program. Subjects underwent a detailed, standardized interview focusing on demographic characteristics and sexual drug use practices. Results indicate that IV drug abusers without AIDS are a homogeneous population of young, poor men and women who are members of minority groups, have a remarkably similar pattern of drug abuse, and who are restricted geographically. Heroin and/or cocaine use was universal, and most had used these drugs for over 5 years. Inoculation of blood via the sharing of contaminated needles appears the most likely feature in the spread of AIDS among IV drug abusers. It remains unclear why some IV drug abusers get AIDS and others do not, since the majority appear to have been exposed to the putative etiologic agent. 3 tables and 23 references.