Background and objetive: Methadone is largely used as the primary opioid substitution therapy for the treatment of heroin addiction; the objective of the study was to describe the clinical characteristics of heroin abusers admitted into a methadone maintenance program (MMP) in metropolitan Barcelona. Method: Cross-sectional study in patients enrolled in MMP since its introduction in 1992 through December 2010. Socio-demographic data, drug use characteristics, prevalence of blood-borne infections (human immunodeficiency virus [HIV], and hepatitis B [HBV] and C [HCV]) and psychiatric co-morbidity were assessed at entry. Results: One thousand and six hundred seventy eight patients (82.8% male). A total of 608 (36.2%) patients were admitted during 1992-1996, 566 (33.7%) between 1997-2001, 305 (18.2%) between 2002-2006 and 199 (11.9%) in the last period. Age at admission to methadone increased significantly (28 years in period 1992-1996 vs. 37 years in the last period [P <.005]). The percentage of patients with a history of intravenous drug use decreased significantly (89.5% in first period vs. 56.4% in period 2007-2010 [P <.05]). Prevalence of HIV, HCV and HBV (HBcAb+) was 53.7, 73.6 and 61.3%, respectively. The prevalence of HIV decreased over time from 66.2% in first period to 43.5% in 2007-2010 (P <.05); the prevalence of HCV decreased significantly from 82.8% in 1992-1996 to 69.8% in last period (P <.05). Twenty five percent of patients had psychiatric co-morbidity at admission and the prevalence of psychiatric co-morbidity increased over time (21% in 1992-1996 and 32% in 2007-2010; P <.05). Conclusion: Age at first opioid substitution therapy is increasing over time, as well as the proportion of patients with psychiatric co-morbidity. There were significant reductions in blood-borne infections.