The 10,010 DATOS clients were interviewed from 1991 to 1993 at entry into treatment in 96 programs in 11 U.S. cities. DATOS used a longitudinal prospective design that focused on providing descriptive information on the characteristics and behaviors of clients as they entered and progressed through treatment. The following categories of measures were used in the regression analyses: predisposing factors, social enabling factors, addiction career characteristics, HIV-risk behaviors, criminal activities, mental health treatment, and treatment career. For half of the clients, DATOS was their first treatment episode; for the other half, their mean number of prior treatment episodes was 3.5. Type of treatment and cumulative length of treatment exposure reflected the interaction of clients' drug-use patterns and the historical availability of different treatment approaches. Regression analyses showed that a higher level of prior treatment use was associated with more severe addiction career characteristics, injection drug use, and criminal activities. Effective and cost-effective approaches to treatment need to include more strategic interventions that consider clients' diverse treatment histories. 4 tables and 37 references