The authors studied four children with Reye's syndrome aged 16 months, 6 years, 8 years and 11 years respectively with severe liver failure and progressive coma. The laboratory investigations in all of them showed a marked elevation of serum transaminases, hyperammoniaemia and a prolongation of the prothrombin time. The electroencephalograms showed a grossly abnormal picture with generalised continuous delta activity. Three children survived with return to normal of the liver function tests whilst the fourth child died. In the last two patients an electroencephalogram carried out every six hours has enabled certain prognostic features to be determined. Improvement in the E.E.G. correlates closely with clinical improvement and vice versa. The authors also advocate serial E.E.G. recordings in Reye's syndrome. The role of hyperammonaemia in the genesis of encephalopathy and the electroencephalographic changed is discussed. The role played by raised intracranial pressure is stressed and the importance of controlling it in order to prevent further damage and improve the prognosis of this serious illness.