Spontaneous, prolonged hypoglycemia was observed in one nondiabetic and one mildly diabetic patient, both with chronic renal insufficiency. Although both patients had abnormal oral glucose tolerance tests, half-time disappearance of glucose was usually normal during intravenous glucose tolerance tests. Both patients developed persistent hypoglycemia during the tolbutamide tolerance test, but the insulin responses were not consistent with an islet cell adenoma. The hypoglycemia did not seem to reflect increased insulin secretion, increased insulin sensitivity or impaired glycogenolysis as assessed by the blood glucose response to glucagon. Although some factor associated with chronic renal disease was probably responsible for the hypoglycemia, the underlying mechanism was not defined. DIABETES 22:493-98, July, 1973.It is well recognized that patients in chronic renal failure develop some degree of abnormal carbohydrate tolerance. 1 " 5 The disturbance in glucose homeostasis most commonly manifests itself as glucose intolerance, referred to as renal diabetes. 1 " 3 -5 ' 6 On rare occasions prolonged and refractory hypoglycemia has been observed independent of a known hypoglycemic stimulus and without adequate explanation. 7 ' 8 Diabetic patients occasionally develop improved carbohydrate tolerance and decreased insulin needs as renal insufficiency develops. 9 " 11 Episodes of severe and prolonged hypoglycemia in diabetic, uremic patients on insulin or oral hypoglycemic agents have also been reported. 12 Recently, spontaneous hypoglycemia was documented in three diabetic patients in renal failure who were being managed by diet alone 7 and in one nondiabetic with a comparable level of renal insufficiency. 8 The latter case is, to our knowledge, the first report of hypoglycemia occurring with renal insufficiency in the absence of diabetes. Recently, we observed hypoglycemia in two patients in