The primary mode for HIV transmission among intravenous drug users (IVDUs) is the sharing of contaminated hypodermic equipment. These activities take place under social conditions that may vary widely among different groups of IVDUs. In 1987, HIV seroprevalence among heterosexual IVDUs was approximately 15 percent in San Francisco and 50 percent in New York City. In New York City 31.6 percent of AIDS cases were diagnosed among heterosexual IVDUs, while 1.3 percent of AIDS cases fell into the same category in San Francisco. Different trajectories of HIV infection among IVDUs in San Francisco and New York are explored; possible explanations for disparities are considered; and the social contexts and conditions in which risk behavior takes place are discussed. Implications for public health policy, community-based interventions, and the efficacy of needle-exchange programs are explored. 4 figures, 10 notes, and 30 references. (Author abstract)