Glycerol, an effective cerebral dehydrating agent, also has gluconeogenic properties, and can thereby elevate serum glucose to dangerously high levels in predisposed patients treated for cerebral edema. The nonketotic hyperosmolar hyperglycemic state usually occurs in cases of maturity onset diabetes or prediabetes, as in the two elderly patients discussed in this paper. The pathogenesis usually evolves through a constant diabetogenic stress that causes persistent hyperglycemia resulting in the exhaustion of ordinarily adequate insulin stores, ultimately allowing hyperglycemia to progress unchecked to metabolic coma. Precautions to recognize this development should be taken in appropriate patients.