The neuropathological aspects of vaccination against pertussis have been analysed. This has been done partly by considering the previous literature and partly by searching for data on any child whose death since 1960 has been thought to have a possible relationship to the vaccination. Twenty-nine in due course were identified on whom a post-mortem examination had been made. Eighteen had died within 3 weeks of inoculation - the remaining eleven had survived the initial illness but had remained retarded, epileptic and disabled. Although a variety of cerebral abnormalities were found, neither those recorded in the present study nor those abstracted from previous case reports, have demonstrated a recurring pattern of inflammatory or other damage which could be accepted as a specific reaction to immunization against whooping cough. Those reactive changes that were occasionally found appear to be indistinguishable from those seen in many other infantile encephalopathies occurring as the result of a hypoxic/ischaemic state supervening on a complex of respiratory complications, fever and convulsions.