Knowledge of prevalence of illegal drug injection can aid the design and evaluation of services for problem drug users. In this study, prevalence of recent injectors in Spain was estimated with a multiplier method using the number of injectors in a population register of new HIV diagnoses, HIV incidence among injectors from cohort studies and HIV prevalence among injectors in a drug treatment register. Prevalence in 2008 was 38.8 (95% CI 23.8-53.8) per 100,000 population, a 2.8 times reduction compared with 2001. This method permits estimation of both prevalence and trends of drug injection. It is sustainable and routinely applicable in many countries.