Spontaneous fasting hypoglycemia developed in four nondiabetic patients with end-stage renal failure. All were undergoing long-term maintenance hemodialysis and three patients were anephric. Hypoglycemia was generally accompanied by severe metabolic acidosis and, in three patients, lactic acidemia. Abnormalities of hepatic structure and/or function were present in three patients. In one patient, hypoglycemia was refractory to exogenous glucagon, failed to respond to alanine, glycerol, or galactose, and was associated with suppressed plasma insulin and elevated plasma glucagon levels. Fasting hypoglycemia appeared to result from several mechanisms. In at least two patients, fasting hypoglycemia and lactic acidosis resulted from impaired hepatic gluconeogenesis in association with impaired or absent renal glucose production. Additionally, substrate limitation probably contributed to hypoglycemia in several patients.