Background: In France, a harm reduction policy was implemented in the late 1980s to reduce the prevalence of HIV and hepatitis infections among people who use drugs (PWUD). The ANRS-Coquelicot survey aims to measure the prevalence of HIV and HCV among PWUD and to understand the determinants of risky behaviors. Methods: Data collection took place in 2002 among 166 PWID recruited from the entire network of specialized drug treatment and harm reduction services in Marseille. Self-reported data were compared with biological data (finger-prick blood on filter paper). Results: HIV prevalence was 22% for both self-reported and biological data. The prevalence of HCV was 52% in self-reported data and 73% in biological data. Overall, 30% of IDUs were unaware of their HCV-positive status. Among those under 30 years of age, 44% of IDUs were already infected with HCV, suggesting that infections occur during the early years of drug use. Conclusion: Harm reduction policies appear to have had an effect on reducing HIV transmission among IDUs, but seem to have a much more limited impact on HCV.