An epidemic of pertussis occurred among hospital staff caring for paediatric patients. Eight physicians and five nurses were affected. Pertussis developed in six newborn infants exposed to infected hospital staff in the nursery. Erythromycin prophylaxis was used to control the epidemic. Clinical pertussis developed in five adults infected with Bordetella pertussis before erythromycin was used, whereas symptoms developed in only one of the eight adults who became infected after erythromycin prophylaxis was started. Pertussis vaccine was given to adult volunteers in the hospital, and in 77% of two hundred and eighty-six vaccinees there was a fourfold rise in pertussis agglutinins. Local reactions were common, and in two vaccinees generalised rashes developed. One of these required treatment with corticosteroids. The risk of pertussis occurring in adults providing medical care for children should be recognised, and employees with symptoms should be removed from the hospital environment.