The prevalence of injection drug use decreased from 67.3% in 1997 to 14.5% in 2006 among Spanish patients infected with human immunodeficiency virus (HIV). A parallel decrease in the prevalence of coinfection with hepatitis C virus was observed, from 73.8% in 1997 to 19.8% in 2006. This steady decrease in the prevalence of coinfection among Spanish patients was caused by a change in transmission routes of HIV infection.