Two children presented with a first episode of diabetic ketoacidosis. Initially both patients made a good clinical and biochemical recovery, but suddenly developed neurological signs consistent with a diagnosis of tentorial herniation. Cranial computed tomography showed signs of cerebral oedema in both cases with evidence of uncal and tentorial herniation in 1 patient, which resolved after the appropriate treatment. The excellent neurological outcome emphasises the need for early recognition and treatment of sudden onset brain oedema in diabetic keto-acidosis.