This article proposes a method of calculating a practical index of improvement for conducting a cost-effectiveness analysis of substance abuse treatment. While the methodology of cost-effectiveness analysis necessitates the use of a single outcome measure, substance abuse treatment can produce multiple, important outcomes that must be considered in policy decisions about funding and alternative treatments. Thus, there is a need to aggregate the variety of outcomes from substance abuse treatment into a single index. The proposed index is a preference-weighted sum of outcomes using data from the Addiction Severity Index. The methodology and use of the index for conducting a cost-effectiveness analysis are illustrated using Addiction Severity Index data from a controlled quasi-experimental field study of case management.