Designing studies to examine hepatitis C virus (HCV) transmission via the shared use of drug injection paraphernalia other than syringes is difficult because of saturation levels of HCV infection in most samples of injection drug users (IDUs). The authors measured the incidence of HCV infection in a large cohort of young IDUs from Chicago, Illinois, and determined the risk of HCV seroconversion associated with specific forms of sharing injection paraphernalia. From 1997 to 1999, serum samples obtained from 702 IDUs aged 18-30 years were screened for HCV antibodies; prevalence was 27%. Seronegative participants were tested for HCV antibodies at baseline, at 6 months, and at 12 months. During 290 person-years of follow-up, 29 participants seroconverted (incidence: 10.0/100 person-years). The adjusted relative hazard of seroconversion, controlling for demographic and drug-use covariates, was highest for sharing "cookers" (relative hazard = 4.1, 95% confidence interval: 1.4, 11.8), followed by sharing cotton filters (relative hazard = 2.4, 95% confidence interval: 1.1, 5.0). Risks associated with syringe-sharing and sharing of rinse water were elevated but not significant. After adjustment for syringe-sharing, sharing cookers remained the strongest predictor of seroconversion (relative hazard = 3.5, 95% confidence interval: 1.3, 9.9). The authors conclude that sharing of injection equipment other than syringes may be an important cause of HCV transmission between IDUs.