In March 2004, an infectious diseases department and a hepatology department in Paris reported to the French Institute for Public Health Surveillance (InVS) the recent occurrence of cases of acute hepatitis C among HIV-positive (HIV+) men who have sex with men. None of the usual risk factors for hepatitis C virus (HCV) transmission (occupational or nosocomial exposure, injection or snorting of drugs, tattooing, etc.) were found in these patients. In July 2004, another Parisian department published a study on 5 cases of acute hepatitis C among HIV-positive gay men diagnosed between 2002 and 2003 during primary or secondary syphilis. To support these clinical observations, an exploratory study of recent cases of acute hepatitis C diagnosed in HIV-positive gay men was initiated by the InVS and conducted between September and October 2004 in the Parisian departments involved in these reports.