The study tested the stratification, market forces, social network, risk-taking syndrome and career intensification hypotheses. Subjects were 366 heroin-addicted felons from Brooklyn. Variables derived from these hypotheses correctly predicted 76 percent of the cases. Hypotheses with the strongest empirical support were the social network and risk-taking syndrome. According to the social network hypothesis, age, gender and ethnic groups developed their own heroin subcultures that influence injecting behavior. In the risk-taking hypothesis, users who engaged in other non-drug reckless behaviors were at a higher risk of injecting heroin. The availability (street price) of heroin was the strongest correlate of heroin injection as more users self-identified as injectors during periods of lower availability. Age and longevity of heroin use negatively correlated with injection. This article discusses policy implications of study findings. Tables, notes, references