Introduction. In France, a policy to reduce the risks associated with drug use was implemented in 1993. Until now, French data on HIV and HCV prevalence among people who use drugs (PWUD) were based on self-reported data. In 2004, the InVS launched a seroprevalence survey among PWID to describe the profiles and practices of PWID, estimate the seroprevalence of HIV and HCV, and evaluate the harm reduction policy. Method. A cross-sectional, multi-city survey (Lille, Strasbourg, Paris, Bordeaux, Marseille) was conducted among PWID who had snorted or injected drugs at least once in their lifetime. A two-stage sampling strategy was implemented (services receiving PWID and PWID). A socio-behavioral questionnaire was administered by an interviewer at the IDU facility, and a self-administered fingerstick blood sample on a filter paper was collected by the IDU. Testing for anti-HIV and anti-HCV antibodies on the filter papers was performed using ELISA tests. The results present estimates that account for the sampling design. Results. A total of 1,462 (61%) IDUs agreed to participate. HCV seroprevalence was 59.8% [95% CI: 50.7–68.3] and 28% among those under 30 years of age. A total of 27% of IDUs mistakenly believed they were HCV-negative. The HIV seroprevalence was 10.8% [95% CI: 6.8–16.6] and 0.3% among those under 30 years of age. HIV/HCV co-infection was 10.2% [95% CI: 6.3–15.9]. In the past month, 13% of IDUs shared syringes, 38% shared small equipment, and 25% shared snorting straws. Discussion-Conclusion. The high prevalence of HCV among young IDUs suggests that infections occur as early as the onset of drug use. High-risk practices persist, creating conditions conducive to the continued transmission of HCV, as well as HIV. (R.A.)