Prolonged cough occurs in a large proportion of the 2 million pilgrims who participate in the annual Hajj in Saudi Arabia. In a prospective seroepidemiological study to determine the incidence of pertussis among 358 adult pilgrims, 5 (1.4%) were found to have acquired pertussis (defined as prolonged cough and a >4-fold increase in the level of immunoglobulin G to whole-cell pertussis antigen). Of the 40 pilgrims who had no pre-Hajj immunity to pertussis, 3 (7.5%) acquired pertussis. Administration of acellular pertussis vaccine to pilgrims before the Hajj should be considered to address this problem.