The relationship among recent life stress, social support, a patient's locus of control, and the control of blood glucose is evaluated in persons with diabetes mellitus, using objective measures of these psychosocial variables. Short-term [fasting blood sugar (FBS)] and long-term [glycosylated hemoglobin (Hgb A-1C)] control measures are taken at two points in time in order to evaluate the effects of the psychosocial variables on change in diabetes control. For life events, a significant positive association was found between the number of recent life events and blood glucose control. Decrease in social support predicted a worsening of longer-term (Hgb A-1C) control over time. An external locus of control within the patient was associated both with poor short-term control at time one and prediction of poorer long-term control over time. The implications of these findings are discussed in support of a biopsychosocial approach to the management of diabetes mellitus.