• A survey was conducted of 89 households in each of which at least one patient with culture-confirmed pertussis had been detected. The source of infection was found to be an adult in 10 (11.2%) of the 89 households, and the rate of secondary attack was 19(10.3%) of 185. Furthermore, a laboratory study disclosed 17 adults with subclinical pertussis; the subclinical infection rate was 17 (25.0%) of 68. When compared with pertussis in young children, the adult illness was generally less severe and had different clinical features. Adult pertussis showed neither leukocytosis nor lymphocytosis, but it produced anti—pertussis toxin anti-body more quickly and higher levels of anti—filamentous hemagglutinin and agglutinin antibodies, and showed stronger growth inhibition of Bordetella pertussis. Although adult pertussis is usually unrecognized because of its different clinical and laboratory features, it is a significant health threat that requires some measures for disease control.