Personality characteristics were assessed in 139 diabetic adults (insulin-dependent diabetes mellitus = 84, non-insulin-dependent diabetes mellitus = 55) to determine whether a relationship existed between glucose regulation and the measured psychological features. Cloninger's Tridimensional Personality Questionnaire (TPQ) was used to evaluate personality while glycated hemoglobin provided an estimate of recent glucose control. Mean glycated hemoglobin (gHb) for the sample was 11.4%. A significant difference (p = 0.03) in gHb levels was observed comparing subjects with extreme personality features (mean = 11.9%) with those subjects with average personality profiles (mean = 10.9%). Post hoc statistical testing identified opportunistic, alienated, and explosive temperaments as the personality features most often associated with poor glucose control. The TPQ appeared to be a particularly useful predictor of poorer glucose regulation in the subset of patients who were self-referred.