Six children had delayed onset of acute posttraumatic subdural effusion. All had suffered an initial head injury for which they had been hospitalized, examined, observed, and then discharged apparently well. All had to be readmitted 16 to 28 days later because of the sudden onset of symptoms and signs indicative of neurological deficit and an acute increase in intracranial pressure. None of the children had a history of recent trauma. All showed angiographic evidence of massive bilateral subdural space-filling defects. Linear skull fracture is considered an "alerting" sign warning the physician to look carefully for either an immediate or delayed acute subdural hematoma. Subdural taps done shortly after the acute injury were of no help in predicting the subsequent course.