Five patients with symptomatic hypoglycemia unresponsive to dietary management were treated orally with 600 mg. of diphenylhydantoin (DPH) daily administered in three divided doses. Treatment with DPH resulted in both subjective and objective improvement in all cases. Serum glucose and insulin levels were determined before and after DPH therapy, and the changes observed after treatment correlated with the clinical response. DPH treatment resulted in an over-all increase of 31 per cent in mean plasma glucose values and an over-all decrease of 76 per cent in plasma insulin levels. These changes are in accord with those previously reported in normal subjects, and they appear to reflect a direct effect of the drug.