Aim: The Research Outcome Study in Ireland (ROSIE) is the first large-scale, prospective, multi-site, drug treatment outcome study in Ireland. Method: Using a structured questionnaire, the substance use, health, crime and social problems of 404 opiate users were assessed at treatment intake (6 months, 1 year and 3 years later). Findings: While the majority of study participants were opiate users, poly-drug use was the norm (76%, n = 308). Most participants had a history of injecting drug use (77%, n = 308), however only 42% (n = 170) reported injecting in the preceding 90 days. Participants reported a range of mental and physical health complaints and extensive contact with social care services. High crime rates were observed. Analysis revealed differences in the characteristics and substance use of participants across treatment modality. Conclusions: The range and severity of problems affecting individuals commencing treatment for their problem drug use highlights the complex needs of the cohort. These problems create substantial costs for providers of social care services in Ireland and can affect treatment outcomes. ; PUBLISHED ; National Advisory Commission on Drugs (NACD)