Two children with idiopathic hypoglycemia of infancy developed diabetes mellitus. One is insulin dependent while the other receives tolbutamide. Despite recurrent hypoglycemia, blood glucose values during oral glucose tolerance were consistent with a diagnosis of diabetes, and the immunoreactive insulin responses to a number of stimuli were severely impaired. Currently accepted glucose-endocrine interrelationships do not explain these observations. Although reports of the progression from hypoglycemia to diabetes in the child are rare, the association appears to be more than a chance occurrence, and warrants further investigation, particularly with regard to the possible role of glucagon.