BACKGROUND: Addiction to heroin and crack cocaine is debilitating and persistent, but such disorders are treatable. We present the first effectiveness study of the main community interventions for addiction to heroin and crack cocaine in England, using data from the National Drug Treatment Monitoring System (NDTMS). METHODS: The study cohort consisted of all adults with a heroin or crack cocaine addiction, or both, who started pharmacological treatment (n=18428 patients) or psychosocial treatment (n=2647) between Jan 1 and Nov 30, 2008, received at least 6 months' treatment or were discharged by the study endpoint (May 31, 2009), and had outcome data submitted to the NDTMS. Effectiveness was assessed from change in days of heroin or crack cocaine use, or both in the 28 days before the start of treatment and in the 28 days before review. FINDINGS: 14656 clients - 74% of the cohort eligible for analysis at review with available data - were analysed at the study endpoint. During the 28 days before review, 37% (5016/13542) of heroin users abstained from heroin and 52% (3941/7636) of crack cocaine users abstained from crack cocaine. A higher proportion of users of heroin only abstained than did users of both heroin and crack cocaine (42% [2465/5863] vs 33% [2551/7679]; OR 1.46, 95% CI 1.36-1.56), and more users of crack cocaine only abstained than did users of both drugs (57% [295/522] vs 51% [3646/7114]; 1.24, 1.03-1.48). Overall heroin use reduced by 14.5 days (95% CI 14.3-14.7) and crack cocaine use by 7.7 days (7.5-7.9). For clients given pharmacological treatment, reduction in days of heroin use was smaller for users of both heroin and crack cocaine than for users of heroin alone (p