In August 1991, the Institute of Medicine released a report entitled Adverse Effects of Pertussis and Rubella Vaccines, which examined, among others, the relation between immunization with whole-cell diphtheria-tetanus-pertussis (DTP) vaccine and both acute encephalopathy and chronic neurological damage. The committee reviewed information from a wide range of both professional and lay sources and found that the evidence is consistent with a possible causal relation between DTP vaccine and acute encephalopathy, although it is insufficient to establish causality. The range of excess risk of acute encephalopathy following DTP immunization is consistent with that estimated from the National Childhood Encephalopathy Study: 0.0 to 10.5 cases per million immunizations. The committee concluded that the evidence is insufficient to indicate either the presence or absence of a causal relationship between DTP vaccine and permanent neurological damage. The evaluative methods used by the committee are briefly described and the evidence underlying its conclusions presented.