Introduction to the special issue: treatment process in DATOS.

Several important findings from the drug abuse treatment outcome studies (DATOS) are presented in this issue of drug and alcohol dependence. These studies focus on the drug abuse treatment process in areas of engagement in treatment and participation in program activities, the effect of the patient's age and treatment history in predicting treatment retention and outcomes, and the impact of prior treatment experience on the level of treatment engagement and subsequent outcomes. A cost-benefit model for drug abuse treatment is developed. Significant contributions are made in the development of a comprehensive model of the treatment process, including the relationship of patient attributes, program factors, and outcomes. Findings on retention from the United Kingdom's national treatment outcome research study (NTORS), a study similar in design to DATOS, also are presented.