The participants were all volunteers registered in the AIDS clinic or the methadone treatment clinic in a large, urban hospital. All had used intravenous drugs and most were currently in treatment in one or both clinics. Forty-six percent of the participants were HIV-positive. Participants completed individually administered questionnaire interviews. The data were analyzed using Pearson correlation coefficients and multiple hierarchical regression analyses. Results revealed that personality, family, and peer characteristics that were related to needle-sharing in females were similar to those in males, with certain exceptions. The role of family, particularly the significant other, was more important and proximal in its effect on needle-sharing behavior in women than in men. Family had both a main effect and a mediating effect in women, buffering risk factors leading to needle sharing. Tables and 35 references (Author abstract modified)