The prevalence of HIV and hepatitis C among prisoners in France was estimated in 2010 based on a cross-sectional study using a two-stage sampling design (sampling of correctional facilities followed by sampling of individuals). The estimates were extrapolated to the entire prison population through post-stratification by age, sex, continent of birth, and region of the facility. Among the 60,975 prisoners in May 2010 across 188 prisons, 2,154 individuals were randomly selected from 27 facilities. HIV prevalence was estimated at 2.0% (95% CI: [0.9–4.2]), and was higher among women (2.6%; 95% CI: [0.7–8.8]) than among men (2.0%; 95% CI: [0.9–4.3]); 75% of HIV-positive inmates were receiving treatment for their infection. HCV prevalence was estimated at 4.8% (95% CI: [3.5–6.5]) and was higher among women (11.8%; 95% CI: [8.5–16.1]) than among men (4.5%; 95% CI: [3.3–6.3]). Nearly half of those infected with HCV had chronic hepatitis (HCV RNA positive) and 44% had received or were receiving treatment for HCV. The prevalence of HIV and HCV was six times higher in the prison population than in the general population, and 2.5% of inmates were viremic (positive viral load) for HCV. Screening and management of these infectious diseases during incarceration are essential to limit transmission and improve the prognosis for these patients. (R.A.)