Two hundred forty-eight subjects were tested before and 5 months after treatment with the Addiction Severity Index (ASI). Exposure to treatment was based on the number of clients' contact hours with a therapist. The sample was divided into three groups according to number of hours spent in treatment. The data were analyzed using MANOVA on the seven scales of the ASI for the three groups and the two time periods. Results showed that the severity of addiction problems decreased after treatment and decreased more for subjects who underwent treatment for a longer period of time. Subjects who underwent a rehabilitation process for longer periods of time had a larger initial deterioration prior to starting treatment than those who quickly withdrew from treatment. Thus, there is a relation between the severity of the problems and the fact of undergoing treatment. Despite the lack of improvement observed for those who left treatment at an earlier time, at the 5-months follow-up these clients showed severity ratings of their problems similar to those who underwent longer rehabilitation. It was impossible to separate those who withdrew from treatment at a favorable moment from those who withdrew prematurely. Tables, figure, notes, references