A biannual survey has been conducted since 1993 in Specialized Residential Treatment Centers for Substance Users (CSSTH). Its objectives are to describe the demographic and substance use characteristics of CSSTH residents and to estimate the prevalence of HIV infection and, since 1996, hepatitis C virus (HCV) infection in this population. This article presents the results for the first half of 1996 and compares them with those from the previous five semesters. (adapted from the text).