Costly complications of diabetes often arise from poor glycemic control. Appropriate diabetes self-care management may improve control. This study examined whether self-care management affects glycemic control and mediates relationships between self-efficacy and self-care agency with glycemic control. In a cross-sectional correlational design, data from a prior study of 141 insulin-requiring adults with type 1 or type 2 diabetes were examined using descriptive statistics, Pearson's correlation, and multiple hierarchical regression. Findings indicated that greater self-care agency and self-efficacy lead to greater self-care management, in turn leading to better glycemic control. Self-care management did not mediate between self-efficacy or self-care agency and glycemic control. Thus, beliefs or capabilities for self-care are insufficient to improve glycemic control; doing so requires self-care management.