Thirty-six children younger than 1 yr of age hospitalized for whooping cough approximately 9 yr previously (cases) were compared with 36 control children of the same age and sex. Subjects were sampled from participants in an earlier large field study of the long-term sequelae of whooping cough. Respiratory symptoms were more common in cases, although the differences were not statistically significant. Cases were significantly more likely either to be atopic or to have a family history of wheezing illness. There were no significant differences between cases and control subjects in lung function indices derived from maximal expiratory flow volume loops or from single-breath nitrogen washout tests or in bronchial reactivity as judged by the histamine challenge PC20. The evidence from both the present and the earlier study indicates that whooping cough is unlikely to be a causal factor in later respiratory illnesses and symptoms and that no deficit in lung function can be detected in later childhood. The disease may, however, occur more frequently or be more easily recognized in children with environmental or constitutional factors that predispose to respiratory morbidity.