Illicit drug injection is a major component of the AIDS epidemic in the United States, Europe and some developing countries. Prevention of illicit drug injection would not only reduce HIV transmission but would also reduce the other health, psychological and social problems associated with illicit drug injection. One hundred and four subjects who were using heroin intranasally ('sniffing') were recruited for a study of the transition to drug injection. Eligibility criteria included sniffing as the most frequent route of administration and no more than 60 injections in the past 2 years. All subjects received thorough basic information about AIDS, including HIV antibody test counseling. Subjects were then randomly assigned to a four-session social learning based AIDS/drug injection prevention program or a control condition. Eighty-three subjects were successfully followed at a mean time of 8.9 months. Twenty (24%) of the followed subjects reported injecting illicit drugs during the follow-up period. Drug injection during follow-up was associated with being in the control group, intensity of non-injected drug use, prior injection, and having close personal relationships with current intravenous (IV) drug users.