In mid-2004, three Parisian hospital wards informed the Institut de veille sanitaire of recent acute hepatitis C in HIV-infected (HIV+) men who had sex with men (MSM). These cases for whom none of the usual bloodborne routes for hepatitis C (HCV) transmission was found, reported having had unprotected sex. In October 2004, we conducted a retrospective investigation in Parisian hospital wards to explore HCV modes of transmission in recent acute hepatitis C in HIV+ MSM. Patient demographics, clinical and biological status of HIV infection, reasons for HCV testing, sexual behaviour and risk factors for HCV transmission within the 6 months before hepatitis onset were collected from medical records. An anonymous self-administered questionnaire on sexual behaviour within the six months before hepatitis onset was also offered to all cases. We identified 29 cases of acute hepatitis C in HIV+ MSM with onset from April 2001 to October 2004. HIV infection was asymptomatic for 76%. Median age at hepatitis C onset was 40 (28-54) years. In all records, were noted unprotected anal sex, fisting in 21% and a concomitant sexually transmitted infection (STI) in 41%. Median time between HIV diagnosis and HCV infection was 6.5 years (0-22). From the 11 self-administered questionnaires completed, 10 reported an STI, 8 'hard' sexual practices, 6 bleeding during sex and 5 fisting. HCV transmission probably occurred through bleeding during unprotected traumatic anal sex among HIV+ MSM and may be facilitated by STI mucosal lesions. This report stresses the continuous need to strongly advocate safer sex to MSM.