Two adult siblings presented with recurrent syncope due to severe hyperinsulinemic hypoglycemia. Exploratory laparotomy in the elder sibling showed a grossly normal pancreas, but histologic examination revealed islet cell hyperplasia. Neither sibling has any evidence of the multiple endocrine neoplasia type 1 syndrome, nor is there any other family history to suggest this diagnosis. To our knowledge, this is the first report of adult-onset familial hyperinsulinism without other manifestations of multiple endocrine neoplasia type 1 syndrome. A simple provocative test for hyperinsulinism was also suggested by these cases. Because the initial patient related his symptoms to exercise, we used treadmill exercise in both patients to diagnose hyperinsulinism and observe its response to therapy.