We set out to examine the impact of treatment for heroin dependence on drug use, injecting behavior, health problems, criminality, and physical and mental health over 18 months among heroin-dependent Londoners. A total of 100 heroin users were recruited for this longitudinal prospective cohort study with repeated measures (T0 as baseline, T1 after 9 months, and T2 after 18 months). The psychiatric evaluation and assessment of drug abuse levels were determined by the CIDI and the EuropASI. Additional evaluations included the WHO-DAS II for disability assessment and the UCLA-SSI for social support. The number of days of heroin use in the 30 days previous to each single assessment significantly reduced over time (p < .001). Similar reduction levels were observed for cocaine (p < .05), benzodiazepines (p < .001), and polydrug abuse (p < .001), but not for cannabis and alcohol. The number of injecting occasions reduced in parallel, with increase in days in work and reduction of money spent for drug acquisition activities and money obtained from criminal/illegal activities. The number of subjects experiencing suicidal ideation reduced over time (p < .05). In line with previous suggestions, significant reductions in drug use, criminality, psychopathology, and injecting behavior following treatment exposure for heroin dependence were observed. It is, however, of concern that alcohol and cannabis misuse levels remained unchanged. (Am J Addict 2012;21:268–273)