Few data have been published on the prevalence of hepatitis C virus (HCV) among injecting drug users (IDUs) in the United Kingdom. This study compares the prevalence of antibody against HCV (anti-HCV) among IDUs in Glasgow in 1990 (when Glasgow's needle/syringe exchange programme had become established) with that in 1995. Serum left over from specimens taken for named HIV antibody testing was tested anonymously for anti-HCV. The prevalence of anti-HCV fell significantly between 1990 and 1995 among IDUs of all ages (90% to 77%), IDUs aged 15 to 19 years (92% to 29%), and IDUs aged 20 to 24 years (91% to 65%). This study suggests that the incidence of HCV infection among young IDUs fell in the early to mid 1990s, after the establishment of Glasgow's needle/syringe exchange scheme between 1988 and 1990. Since almost a third of injectors under 20 years of age when tested in 1995 had been infected with HCV, however, other interventions may be needed to prevent the spread of HCV in this high risk group.