It has been speculated that stressful life events can precipate some autoimmune diseases by altering the immune system. This study was undertaken to test this hypothesis in type I (insulindependent) diabetes. Thirty-two young patients (less than 40 years) with a recently diagnosed IDDM and 53 age-matched controls were interviewed according to a standardized questionnaire designed to identify, date, and weigh past stressful life events. In patients immunological status was assessed during the six months following the first manifestation of the disease by measuring anti-organ and anti-islet cell antibodies, T lymphocyte subsets, PHA mitogenic activity, IL2 production by blood mononuclear cells, IgG, IgA, IgM, and C3, C4 component fraction levels. The diabetic population experienced fewer life events, stressful or non-stressful, but in the 12 months preceding the onset of the disease, 50% of the diabetics endured at least one stressful life event as against only 18.8% of the controls (p less than 0.01). The only difference in the immunological status of the patients who had experienced a stressful life event in the previous twelve months and those that had not, involved PHA mitogenic activity which was significantly lower after a stressful event. While these findings do attest to a temporal relation between stress and type I diabetes in at least 1 out of 2 patients, they do not establish a causative connection.