Background Since 2000, hepatitis C virus (HCV) infection, associated with high risk sexual behaviour, has emerged as an STI among HIV infected MSM. We aimed to identify individual and network-related factors associated with HCV infection among HIV infected MSM. Methods The study population was recruited at the STI outpatient clinic of the public health service and the HIV Treatment Centre of the Academic Medical Centre, Amsterdam, the Netherlands. Inclusion criteria were male gender, having had sexual contact with men in the previous 6 months, age ≥18 years, and understanding of written Dutch or English. Recruitment occurred from July 2008 to August 2009. Participants were screened for chlamydia, gonorrhoea, syphilis, and hepatitis B. HIV testing was done using an opting-out strategy and HIV infected MSM were tested for the presence of HCV antibodies. Participants completed a questionnaire including demographics and detailed questions about sexual behaviour in self-defined relationships with a steady partner and the most recent three other partners within the previous 6 months. Logistic regression analysis was used to identify factors associated with HCV infection. Results 850 HIV infected MSM reporting 2290 relationships were included in the study, of whom 108 were HCV antibody positive (overall prevalence 12.7%). MSM who reported a history of injecting drug use (n=3) were excluded. In multivariate analysis, independent risk factors for HCV infection were unprotected receptive anal intercourse (OR 1.60 95% CI 1.00% to 2.57%) and reporting group sex (OR 1.85 95% CI 1.09% to 3.15%). Drug use (ie, XTC, GHB, or ketamine) was also associated with HCV (OR 2.28 95% CI 1.33% to 3.92%). There was significant interaction between fisting and self-described lifestyle type of MSM: the OR of non-fisting, leather/rubber/lycra type MSM was 4.22 (95% CI 2.22% to 8.00%); the OR of fisting non-leather/rubber/lycra type was 2.01 (95% CI 1.00% to 4.04%); and the OR of fisting leather/rubber/lycra type was 2.71 (95% CI 1.37% to ...