This paper reports changes in substance use behaviours at 1-year follow-up, and investigates the relationship between time in treatment and observed outcomes. A total of 408 clients were interviewed at intake to 23 residential treatment programmes, and 286 (70%) of these were interviewed at 1 year. Substantial improvements were found in terms of abstinence from opiates, psychostimulants and benzodiazepines. At 1 year, half of the clients were abstinent from heroin. Reductions in injecting, sharing injecting equipment, heavy drinking and criminal behaviour were found. Critical treatment thresholds were identified using multiple logistic regression analyses. Longer stays in treatment were predictive of better 1 year outcomes.