In persistent hyperinsulinemic hypoglycemia of infancy (PHHI), the long term outcome of the disease is not well documented. Previous reports suggested that partial pancreatectomy in infants does not endanger future islet function. We evaluated endocrine pancreatic function in 14 PHHI patients 6.5-21 yr after diagnosis. Eight underwent early subtotal pancreatectomy, and 6 were medically treated; all were in clinical remission. Intravenous glucose tolerance and C-peptide suppression tests were performed, with multiple determinations of hormone levels. The insulin response to glucose was blunted in all pancreatectomized and in 2 conservatively treated patients. Glucose disposal was reduced in 6 pancreatectomized patients and in 2 medically treated subjects. Six of the pancreatectomized patients, including two with normal glucose disposal at first evaluation, developed overt diabetes during puberty. None in the medically treated group became diabetic; however, only 2 patients have reached puberty. C-Peptide suppression in response to hypoglycemia was inadequate in 4 of 5 pancreatectomized and 3 of 5 nonpancreatectomized patients studied. These results show that children with PHHI have impaired insulin responses to glucose and lack of suppressibility of endogenous insulin secretion several years after clinical remission. Thus, the beta-cell defect responsible for the disease in infancy is not corrected with time despite the disappearance of spontaneous hypoglycemia. Insulin secretion seems more disturbed in pancreatectomized patients; the majority develop insulin-requiring diabetes during puberty. An effort should be made to treat PHHI patients medically to avoid pancreatectomy; this may reduce the incidence of diabetes at puberty.