Data from large-scale registers is often underutilized when evaluating addiction treatment programs. Since many programs collect register data regarding clients and interventions, there is a potential to make greater use of such records for program evaluation. The purpose of this article is to discuss the value of using large-scale registers in the evaluation and program planning of addiction treatment systems and programs. Sweden is used as an example of a country where register data is both available and is starting to be used in national evaluation and program planning efforts. The article focuses on possibilities, limitations and practicalities when using large-scale register data to conduct evaluations and program planning of addiction treatment programs. Main conclusions are that using register data for evaluation provides large amounts of data at low cost, limitations associated to the use of register data may be handled statistically, register data can answer important questions in planning of addiction treatment programs, and more accurate measures are needed to account for the diversity of client populations.