Young injection drug users (IDUs) in San Francisco may be at high risk for hepatitis C virus (HCV) infection despite access to several needle exchange venues. The authors conducted a cross–sectional study from 1997 to 1999 in San Francisco to estimate the prevalence and incidence of antibody to HCV (anti–HCV) among street–recruited IDUs under age 30, and to examine risk behaviors and sources of sterile needles. Among 308 participants, the prevalence of anti–HCV was 45%. Using statistical modeling, incidence of HCV infection was estimated to be 11 per 100 person years. Independent risk factors for anti–HCV included age (odds ratio [OR], 1.17 per year; 95% confidence interval [CI], 1.05–1.30), years injecting (OR, 1.21 per year; 95% CI, 1.10–1.34), years in San Francisco (OR, 1.06 per year; 95% CI, 1.00–1.14), first injected by a sex partner (OR, 4.06; 95% CI, 1.74–9.52), injected daily (OR, 3.85; 95% CI, 2.07–7.17), ever borrowed a needle (OR, 2.56; 95% CI, 1.18–5.53), bleached last time a needle was borrowed (OR, 0.50; 95% CI, 0.24–1.02), snorted or smoked drugs in the prior year (OR, 0.48; 95% CI, 0.26–0.89), and injected by someone else in the prior month (OR, 0.50; 95% CI, 0.25–0.99). In the prior month, 88% used at least 1 of several needle exchange venues, and 32% borrowed a needle. We conclude that anti–HCV prevalence is lower than in previous studies of older IDUs, but 11% incidence implies high risk of HCV infection in a long injecting career. Despite access to sterile needles, borrowing of needles persisted.