Forty-three patients with burns of various depth and burn surface area were studied to determine the relationship between burn severity and changes of serum osmolality. The results showed that the osmolality in four patients who died were significantly higher than the osmolalities of survivors with very serious burns, whose osmolalities decreased gradually to normal level. Hyperosmolality and obvious osmolar discrepancy often co-exist. It is suggested that the changes of osmolality and osmolar discrepancy can reflect the burn severity and prognosis of the patients.