The first 1000 cases notified to the National Childhood Encephalopathy Study were analysed. The diagnoses included encephalitis/encephalopathy, prolonged convulsions, infantile spasms, and Reye's syndrome. Eighty-eight of the children had had a recent infectious disease, including 19 with pertussis. Only 35 of the notified children (3.5%) had received pertussis antigen within seven days before becoming ill. Of 1955 control children matched for age, sex, and area of residence, 34 (1.7%) had been immunised with pertussis vaccine within the seven days before the date on which they became of the same age as the corresponding notified child. The relative risk of a notified child having had pertussis immunisation within that time interval was 2.4 (p less than 0.001). Of the 35 notified children, 32 had no previous neurological abnormality. A year later two had died, nine had developmental retardation, and 21 were normal. A significance association was shown between serious neurological illness and pertussis vaccine, though cases were few and most children recovered completely.