To determine whether traits of normal personality are associated with variations in glycemic control in patients with type 2 diabetes. A longitudinal cohort study was conducted using data from 105 type 2 diabetic patients in a clinical trial of a stress management intervention. Before treatment assignment, patients completed the NEO Personality Inventory, Revised, which is a questionnaire inventory measuring 5 major domains of normal personality and 30 important traits that define these domains. Glycemic control was assessed by measures of HbA1c and average blood glucose levels based on 7 days of self-monitoring at baseline and at 6 and 12 months. Relationships between personality traits and measures of glycemic control were examined by correlation and linear regression models that were adjusted for age, sex, race, duration of diabetes, medication status, and experimental treatment. Lower average blood glucose values at baseline were associated with higher scores for the personality domain of neuroticism and several specific traits including anxiety, angry hostility depression, self-consciousness, and vulnerability but were associated with lower scores for the trait of altruism. Results were similar for HbA1c but were not as strong. Follow-up results were similar but were less consistent. Personality traits may offer new insights into variations in glycemic control in patients with type 2 diabetes undergoing standard management. The relative tendency to experience fewer negative emotions and to focus on the needs of others instead of oneself could prove to be a risk factor for poor glycemic control.