We have attempted to identify any characteristics which could be used to predict the development of cerebral edema in four children under 5 years of age with new onset insulin-dependent diabetes mellitus and diabetic ketoacidosis. We retrospectively analysed and compared the concentration of serum sodium (corrected for serum glucose value) and effective serum osmolality of these 4 children with values of 10 age-matched controls with new onset insulin-dependent diabetes mellitus who did not develop cerebral edema during treatment of diabetic ketoacidosis. The initial serum sodium values of the two groups were not statistically different. Patients who developed cerebral edema had lower initial serum glucose values and effective serum osmolality. During treatment, patients who developed cerebral edema had consistently lower mean serum sodium and osmolality than controls at each 4-h interval after the first 4 h of therapy. Serum sodium and osmolality declined progressively after the initiation of therapy in cerebral edema patients, while remaining stable in controls. These data suggest that children who develop cerebral edema during treatment for diabetic ketoacidosis initially may have a relatively normal serum osmolality and subsequently develop progressive hyponatremia and/or a trend of declining serum sodium before developing cerebral edema.