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. 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. 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. 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.