The cost of treating approximately 150,000 participants represented by the CALDATA study sample in 1992 was $209 million. Each day of treatment paid for itself on the day it was received, primarily through an avoidance of crime. The benefits of alcohol and other drug treatment outweighed the costs of treatment by ratios from 4:1 to greater than 12:1, depending on the type of treatment. Benefits after treatment persisted through the second year of follow-up for the limited number of participants followed for as long as 2 years. This suggests that projected cumulative lifetime benefits of treatment will be substantially higher than the shorter-term figures. The level of criminal activity declined by two-thirds from before treatment to after treatment. The greater the length of time spent in treatment, the greater the percentage of reduction in criminal activity. Declines of approximately two-fifths also occurred in the use of alcohol and other drugs from before treatment to after treatment. Approximately one-third reductions in hospitalizations were reported from before treatment to after treatment. For each type of treatment studied, there were slight or no differences in effectiveness based on gender, age, or ethnicity. Overall, treatment did not have a positive effect on the economic situation of the participants during the study period. 38 tables and 11 figures