The Addiction Severity Index (ASI) is a semistructured interview that collects data from substance abusers in seven problem areas: medical, employment, legal, alcohol, other drug use, family-social functioning, and psychological status. In each area, the clients provide an estimate of the seriousness of the problem and their need for treatment. It has been demonstrated in a number of studies to be reliable, valid, and useful in monitoring treatment changes and in matching substance-abusing clients to treatments. This study investigates the usefulness of the ASI with male and female substance abusers who suffer a concurrent major psychiatric disorder (N= 152). Data on the independence of the problem scales, their internal consistency, interrater reliability, and concurrent validity are presented. It is concluded that: a) the problem areas are independent from each other, underscoring the need for multidimensional assessment; b) interviewer severity ratings provide information in addition to that provided by clients; c) the reliability of the composite scores is generally adequate, with the exception of the legal and family-social scales; and d) further examination of the employment scores is required.