Identification of relevant individual and intervention variables whose interaction affects response to treatment can be used to facilitate favorable outcomes through appropriate client-treatment matching. To provide a preliminary appraisal of this interaction hypothesis by examining the relationship between client and treatment loci of control as a predictor of success in weight control programs, questionnaires were administered to 23 college student subjects during individual interviews in which their prior experiences in weight control programs (a prerequisite for participation) were also evaluated. The obtained information permitted classification of subjects and their prior weight control programs as being primarily either internally-oriented or externally-oriented. Analysis of weight loss data revealed a significant interaction between subject and treatment loci of control. The mean weight loss per week by the 11 subjects whose internality-externality orientation was similar to that of their treatment was 2.52 pounds, more than 2 1/2 times the 0.97 pounds per week lost by the 12 subjects whose orientation contrasted with that of their treatment. The findings suggest the potential value of efforts to identify relevant client and treatment characteristics that are useful for therapeutic prescription. (Author/JAC)