Treatment is effective in reducing heroin use and clinical and social problems among heroin addicts. The effectiveness is related to the duration of treatment. "VEdeTTE" is an Italian longitudinal study funded by the Ministry of Health to evaluate the effectiveness of treatments provided by the National Health Services. The study involved 115 drug treatment centers and 10,454 heroin users. Clinical and personal information were collected at intake through a structured interview. Treatments were recorded using a standardized form. Survival analysis and Cox Proportional Hazard model were used to evaluate treatment retention. Five thousand four hundred and fifty-seven patients who started a treatment in the 18 months of the study were included in the analysis: 43.2% received methadone maintenance therapy (MMT), 10.5% therapeutic community, and 46.3% abstinence-oriented therapy (AOT). The likelihood of remaining in treatment was 0.5 at 179 days. The median daily dose of methadone was 37 mg. Psychotherapy was provided in 7.6% of patients receiving methadone and 4.9% of those in therapeutic community. Type of therapy was the strongest predictor of retention, with AOT showing the lowest retention. In MMT patients, retention improved according to dose. Living alone, psychiatric comorbidity and cocaine use increased the risk of dropout. Psychotherapy associated halved the risk of dropout.