Injection drug users represent the largest cohort of patients with established hepatitis C virus (HCV) infection as well as the group that is at highest risk for new infections. Most published studies have focused on the clinical consequences of established HCV infection and have not examined the consequences of new infection. The aim of the current study was to measure the virological consequences of HCV in patients with ongoing injection drug use that might pose a risk for new and/or for superinfection with additional strains of HCV. We examined the following groups: (a) those with resolved HCV infection with ongoing injection drug use, (b) those with chronic infection who continued to inject and (c) those with chronic infection who no longer injected. Our study demonstrated a spectrum of responses. The majority of patients appeared to be 'protected' from new infection. None of six patients with resolved infection had detectable HCV RNA by quantitative or qualitative PCR when followed for 1 year. Similarly, despite ongoing injection drug use, no patient with persistent infection had a 'switch' in HCV genotype indicative of possible superinfection. Virological analysis of HCV quasispecies to detect possible infection with new variants of HCV in patients with apparently 'stable' infection, indicated divergence of virus over time, divergence that was unrelated to injection drug behaviour. Thus, patients with ongoing or prior HCV infection appear to develop immunity that protects against further infection with HCV despite repeated exposure.