Twelve cases of diabetic hyperosmolar coma without ketosis are described in ten patients. Seven of these were complicated by other illnesses. Seven died, but in five death Was probably not due to the hyperosmolar state. One had a pancreatic carcinoma; others in the literature are reported to have had pancreatitis. The hyperosmolarity in this syndrome is due to either gross hyperglycemia or hyperelectrolytemia or both. Focal or generalized seizures occur in some cases. Very hypotonic neutral fluid in Targe quantities appears to be essential in the treatment in addition to insulin. No case has been reported in a young person and all patients have been either new diabetics or their diabetes has been previously mild, and remains mild in those who live. The absence of ketosis is possibly due to a block in the metabolic pathway of fatty acid breakdown. Hyperosmolarity must also be considered as a factor in the coma associated with ordinary diabetic ketosis.