Background Since 2000, there has been a marked rise in acute hepatitis C virus (HCV) in HIV-positive men who have sex with men (MSM). We conducted an international phylogenetic study to investigate the existence of an HCV transmission network among MSM. Methods HIV-positive MSM diagnosed with recent HCV (n=226) in England (107), Netherlands (58), France (12), Germany (25) and Australia (24) between 2000 and 2006 were enrolled into a molecular phylogenetic study. Using real-time PCR, the NS5B region of the HCV genome (436 bp) was amplified, sequenced and compared with unrelated NS5B sequences. Results NS5B sequences were obtained from 200 (89%) cases. Circulating HCV genotypes were 1a (59%); 4d (23%), 3a (11%), 1b (5%), and 2b/c (3%). Phylogenetic analysis revealed 156 (78%) sequences that formed 11 clusters (bootstrap value >70%) containing between 4 and 37 individual sequences. Country mixing was associated with larger cluster size (17 vs 4.5 sequences; p=0.03). ‘Molecular clock’ analysis indicated that the majority (85%) of transmissions occurred since 1996. Discussion Phylogenetic analysis revealed a large international network of HCV transmission among HIV-positive MSM. The rapid spread of HCV among neighboring countries is supported by the large proportion (74%) of European MSM infected with an HCV strain co-circulating in multiple European countries; the low evolutionary distances among HCV isolates from different countries; and the trend toward increased country mixing with increasing cluster size. Temporally, this epidemic coincides with the introduction of highly active anti-retroviral therapy and associated increases in sexual risk behaviors. International collaborative public health efforts are needed to mitigate HCV transmission among this population.