A series of 42 children is described who, following a seemingly minor or trivial head injury, developed neurological signs after a lucid or symptom-free period. This group constitutes 4.34 per cent of 967 consecutive patients aged 2 months to 17 years who were seen by members of the neurological staff during the years 1978-1981. Only one patient had an intracranial haematoma. The majority of patients showed a benign transient syndrome consisting of either convulsive or nonconvulsive signs with a spontaneous and full recovery. There were, however, 3 deaths in this series, apparently due to severe and uncontrollable unilateral or diffuse brain swelling, demonstrating the malignant counterpart of this benign syndrome. The theories seeking to explain these phenomena are reviewed. Special reference is made to the hypotheses of Bruce and his associates regarding brain swelling as a causative factor. It is considered that an adequate theory to explain the pathogenesis is still lacking. It is concluded that the juvenile brain responds to cranial trauma in a manner different from the adult brain. This implies a different approach in policy to hospital admission.