This report provides a detailed account of the steps involved in setting up and conducting a pilot study of drug users carried out between January 2001 and June 2002 in the city of Marseille. This study first sought to determine the feasibility of a larger-scale cross-sectional data collection (multi-city, multi-site) whose scientific objectives were to describe the characteristics, needs, and risk-taking behaviors of drug users, and to measure the prevalence and incidence of HIV and HCV using biological samples (saliva or blood) within this population. This feasibility phase also provided epidemiological and biological data on a sample of 166 individuals, the analysis of which is included in the report.