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.