Although pertussis in adults is well documented, opinions differ about incidence of adult disease and about the role of adults as reservoirs of infection. We made use of a prospective household contact study of an acellular pertussis vaccine to collect data about pertussis in adults. All members of families with an index case of pertussis were monitored for respiratory symptoms, and pertussis was confirmed by laboratory tests. In 122 households, 104 children (85%) and 18 adults (15%) were the source of pertussis. These households consisted of 265 adults (aged 19-83 years), in 84 of whom (31%) pertussis was confirmed. Of these 84, 81% had respiratory symptoms for 21 days or more. The spread of pertussis was independent of whether a child (74/104) or an adult (14/18) was the index case. Most adult index cases had no pertussis recall (odds ratio 11.8). The overall attack rate in adult contacts was 0.267 and was independent of the social status and the size of the family and of a pertussis recall, although it differed significantly between women and men (p < 0.05). Erythromycin treatment of the index case reduced the attack rate significantly (p < 0.05). Patients whose first pertussis episode dated back more than 20 years had similar symptoms and attack rates to patients without a recall. We conclude that adults are often involved in the spread of pertussis, and that they can be susceptible to reinfection 20 years after a first pertussis episode.