Retention in treatment is a key indicator of methadone treatment success. The study aims to identify factors that are associated with retention.To determine retention in treatment at 12 months for Irish opiate users in methadone substitution treatment and to indicate factors that increase the likelihood of retention.National cohort study of randomly selected opiate users commencing methadone treatment in 1999, 2001, and 2003 (n = 1269).Sixty-one percent of patients attending methadone treatment remained in continuous treatment for more than 1 year. Retention in treatment at 12 months was associated with age, gender, facility type, and methadone dose. Age and gender were no longer significant when adjusted for other variables in the model. Those who attended a specialist site were twice as likely to leave methadone treatment within 12 months compared with those who attended a primary care physician. The most important predictor of retention in treatment was methadone dose. Those who received <60 mg of methadone were three times more likely to leave treatment.Retention in methadone treatment is high in Ireland in a variety of settings. The main factors influencing retention in methadone treatment was an adequate methadone dose and access to a range of treatment settings including from primary care physicians.Providing an adequate dose of methadone during treatment will increase the likelihood of treatment retention. Methadone treatment by the primary care physician is a successful method of retaining opioid users in treatment.