Heroin overdoses increased sharply in the US in the 1990s, but few studies have addressed overdose risk. We examined overdosing and injection-related risk behavior in young injection drug users (IDUs). We interviewed all consenting injectors under age 30 at needle exchanges and youth outreach sites in San Francisco. Their median age was 22, and their median number of years of injecting was 4. About 48% reported at least one overdose, with a median of two overdoses reported. Overdosing was associated with injecting "speedballs" (i.e. mixtures of heroin and cocaine), with borrowing syringes, and (with P-values of borderline statistical significance) with heroin injection and with gay or bisexual behavior. It was not associated with age, sex, years of injecting, or frequency of injecting. In multivariate analysis, only borrowing syringes and gay or bisexual behavior were independent statistically significant predictors, probably because gay and bisexual subjects were more likely to be heroin or "speedball" injectors. Most subjects (65%) reported that they had not received medical attention at time of last overdose. Risk of overdose in young injectors is acute and closely associated with HIV risk. HIV interventions should include overdose prevention. Emergency response protocols should minimize risk of arrest. Injectors and providers should be trained in overdose prevention, and developing overdose interventions should be a priority among drug educators.