Seventy-eight critically ill patients who died while on the neurosurgical service were studied retrospectively to establish the prevalence of nonketotic hyperglycemic hyperosmolar coma (NHHC). All the patients had been comatose before death, and all underwent necropsy. Criteria for the diagnosis of NHHC included moderate-to-severe hyperglycemia with glucosuria, absence of significant acetonuria, hyperosmolarity with dehydration, and neurological dysfunction. This study revealed seven cases of unequivocal NHHC (9%), and six of hyperosmolarity but with incomplete records. Five of the seven confirmed cases of NHHC demonstrated no evidence of cerebral edema transtentorial herniation, or brain-stem damage, and showed central nervous system (CNS) lesions compatible with survival. Fatal complications of this syndrome, such as acute renal failure, terminal arrhythmias, and vascular accidents, both cerebral and systemic, were common in this series. The mechanism of coma in NHHC is believed related to shifts of free water from the cerebral extravascular space to the hypertonic intravascular space, with subsequent intracellular dehydration, accumulation of metabolic products of glucose, and brain shrinkage. It is uncertain whether injury to specific areas in the CNS is a predisposing factor to the development of NHHC. Factors documented to be significant in its development include nonspecific stress to primary illnesses, hyperosmolar tube feedings, dehydration, diabetes and mannitol, Dilantin, or steroid administration.