The benefit of pertussis vaccine in the control of pertussis is described in relation to the changes in incidence of pertussis during the past thirty years in the U. K. The current large epidemic of pertussis since November 1977 followed a decline in the acceptance of pertussis vaccine (DTP) from an average level of 70 to 80% of children in different areas in 1973 to less than 40% in 1976. The hazards of vaccine range from minor local to systemic (febrile) reactions. Convulsions, infantile spasms and more serious neurological illnesses have been described following inoculation. Serious neurological illnesses have been reported retrospectively and their causation in relation to pertussis vaccine inoculations is doubtful. They do not differ from similar illnesses in children of the same age which have developed without relationship to inoculations. Prospective studies of reactions in recently inoculated children are described. A national study of encephalopathy in babies and infants admitted to hospital in Great Britain is now taking place. Public acceptance of vaccine is hindered by publicity concerning the risks of vaccine and the need exists for improved health education concerning the merit of immunization.