AIM: To determine 1 year outcomes for drug use, criminality, psychopathology and injection-related health problems in those entering treatment for heroin dependence in Australia. DESIGN: Longitudinal prospective cohort study. PARTICIPANTS: Seven hundred and forty five individuals entering treatment (methadone/buprenorphine maintenance therapy; detoxification; residential rehabilitation) and 80 heroin users not seeking treatment. SETTING: Sydney, Melbourne and Adelaide, Australia. FINDINGS: A total of 657 individuals were re-interviewed at 1 year, 80% of the original sample. There were substantial reductions in heroin and other drug use across all three treatment modalities. The majority of those who had entered treatment were heroin abstinent at 1 year (maintenance therapy 65%, detoxification 52%, residential rehabilitation 63%) compared to 25% of the non-treatment sample. The reduction in heroin use among the treatment samples was paralleled by reductions in poly drug use. There were also substantial reductions in risk-taking, crime and injection-related health problems across all treatment groups, and less marked reductions among the non-treatment group. Psychopathology was dramatically reduced among the treatment modalities, while remaining stable among the non-treatment group. Positive outcomes at 1 year were associated with a greater number of cumulative treatment days experienced over the 1 year follow-up period ('treatment dose') and fewer treatment episodes undertaken in that time ('treatment stability'). CONCLUSIONS: At 1 year, there were impressive reductions in drug use, criminality, psychopathology and injection-related health problems following treatment exposure. The positive findings were associated with a greater "dose" of treatment, and with more treatment stability over the follow-up period.