OBJECTIVE: In this paper, we review evidence of HCV prevalence among injecting drug users (IDUs) worldwide. METHODS: We undertook a desk-based review of both 'grey' and published literature released between 1998 and 2005. RESULTS: Data on HCV prevalence among IDUs was found in 57 countries and in 152 sub-national areas. We found reports of HCV prevalence of at least 50% among IDUs in 49 countries or territories. Available regional estimates varied widely, from 10 to 96% in Eastern Europe and Central Asia, from 10 to 100% in South and South-East Asia, from 34 to 93% in East-Asia and the Pacific, from 5 to 60% in North Africa and the Middle-East, from 2 to 100% in Latin America, from 8 to 90% in North America, from 25 to 88% in Australia and New Zealand, and from 2 to 93% in Western Europe. Only in Colombia and Lebanon were all HCV prevalence estimates below 20%. In addition, evidence of HIV/HCV co-infection among IDUs was found in 16 countries. In China, Poland, Puerto Rico, Russia, Spain, Switzerland, Thailand and Viet Nam, estimates of the prevalence of HIV/HCV co-infection among IDUs reached 90%. DISCUSSION: Taken together, data suggest high global prevalence of HCV and HIV/HCV co-infection among IDUs. We suggest exploring protective factors in sites of low HCV prevalence.