Objectives Investigate effect of baseline motivation for change on treatment fidelity, therapeutic alliance, treatment dose, and treatment outcome in a randomized controlled trial of family therapy for youth with poorly controlled diabetes. Methods Seventy-four adolescents and caregivers completed measures of motivation for change. Measures of fidelity, alliance, dose, and youth health status were collected. Structural equation modeling was used to test the direct and indirect effects of motivation on treatment outcomes. Results Parent motivation was significantly related to alliance and fidelity. Only alliance was significantly related to posttreatment metabolic control. In adolescent models, only motivation was significantly related to alliance. In both models, motivation had a significant indirect effect on metabolic control through alliance. Conclusions Findings demonstrate the importance of parent and youth initial motivational status and treatment alliance to treatment outcome among youth with poorly controlled diabetes. Additional research on treatment techniques that promote motivation for change is needed.