DKA-hyperosmolar coma is a readily diagnosed and easily treated, potentially catastrophic emergency that regularly occurs in both Type I and Type II diabetics. This review emphasized that diabetic ketoacidosis and hyperosmolar coma can, and very frequently do, occur concurrently, but it is the hyperosmolar state rather than the DKA that is the primary cause of coma and death in this condition. One must therefore vigorously treat the hyperosmolarity and resulting dehydration, especially when total calculated osmolarity exceeds 230 to 240 mOsm/L. The major aim of treatment is to rapidly replace the major water loss that is responsible for this clinical condition and to stimulate glucose metabolism with insulin. The diagnosis of this dangerous condition is relatively simple. The therapy, in most regards, is equally apparent. There are good data demonstrating that the prompt recognition of DKA-hyperosmolar coma and the simple institution of rapid rehydration have continued to reduce the mortality and complications of this potentially disastrous complication of diabetes mellitus.