Objective: Grounded in Self-Determination Theory, this study examines the role of parental expectations and communication style (ie, in an autonomy-supportive vs controlling way) in the prediction of adolescent motivation (ie, internalization or defiance) to adhere to self-management for type 1 diabetes. Methods: Structural Equation Modeling was used in a cross-sectional, multi-informant study of 129 adolescents (Mage = 14.43; 54.4% girls), 110 mothers, and 98 fathers. Adolescents reported on self-motivation, treatment adherence, and parental expectations and communication styles; parents reported on their own expectations, communication style, and perceptions of adolescent treatment adherence. Medical record review provided HbA1c values. Results: Across adolescent and parent reports, parental communication of diabetes-specific expectations and an autonomy-supportive style of communicating expectations related positively to adolescents' internalization of diabetes self-management and negatively to defiance against diabetes self-management. In contrast, a controlling parental communication style showed the opposite patterns of associations. Higher adolescent defiance was related to poorer treatment adherence and worse glycemic control. Conclusions: Parental communication styles related to adolescent motivation, which in turn, related to adolescent treatment adherence and glycemic control. Future longitudinal research can address the long-term impact of both maternal and paternal communication styles on adolescent motivation to adhere to treatment and their subsequent glycemic control.