Key points: - The Coquelicot study aims to test the feasibility of collecting blood samples from intravenous drug users (IDUs) using finger-prick blood collection on filter paper, to measure the prevalence of HIV and HCV among IDUs in Marseille, and to understand the factors driving risky behaviors. - Data collection for the pilot study took place in 2002 at 15 specialized drug treatment and harm reduction centers and in 9 private medical practices. - HIV prevalence was 22% among IDUs. HCV prevalence was 73% among IDUs and 44% among those under thirty. Overall, 30% of IDUs were unaware of their HCV-positive status. - Harm reduction policies appear to have had an effect on reducing HIV transmission among IDUs, but seem to have had a more limited impact on HCV. (R.A.)