Patients with idiopathic postprandial syndrome (IPS) report recurrent postprandial episodes that resemble the clinical manifestations of hypoglycemia. In an effort to find objective criteria for diagnosis of IPS, we studied a group of patients with IPS and controls during an oral glucose tolerance test. Patients with IPS had a significantly lower mean glucose nadir and higher hypoglycemic index than controls. Although 74% of patients with IPS had glucose nadirs higher than 50 mg/dL, their responses of epinephrine, cortisol, growth hormone, glucagon, and norepinephrine were significantly higher than the respective changes in the controls. Of the five hormones, only the epinephrine response separated the patients from the controls without overlap. The epinephrine response may represent a valuable diagnostic criterion for this disorder.