A longitudinal study tested the self-determination theory (SDT) process model of health behavior change for glycemic control within a randomized trial of patient activation versus passive education. Glycosylated hemoglobin for patients with Type 2 diabetes (n=159) was assessed at baseline, 6 months, and 12 months. Autonomous motivation and perceived competence were assessed at baseline and 6 months, and the autonomy supportiveness of clinical practitioners was assessed at 3 months. Perceptions of autonomy and competence were promoted by perceived autonomy support, and changes in perceptions of autonomy and competence, in turn, predicted change in glycemic control. Self-management behaviors mediated the relation between change in perceived competence and change in glycemic control. The self-determination process model fit the data well. (c) 2004 APA, all rights reserved