Four children presented with cerebral oedema secondary to diabetic ketoacidosis, each of whom had a different clinical picture and time of onset of neurological deterioration. No single factor emerged as the cause of the cerebral oedema, but disturbances in brain water balance appeared to be already operative at the time of presentation with ketoacidosis. Irregularities in treatment may exacerbate these disturbances, leading to frank cerebral oedema. Diagnosis of this dangerous complication of diabetic ketoacidosis depends on clinical awareness; the diagnosis may be confirmed by CT scan. Management remains empirical.