Intrafamilial spread of pertussis was evaluated in 21 families (97 individuals) of patients with whooping cough diagnosed by culture or by ELISA serology. During follow-up (average six months), an infectivity rate of 83% was established by an ELISA within these families. However, 46% of the secondary cases were asymptomatic. Most of the asymptomatic cases were in adults or vaccinated children. Unvaccinated infants had classic whooping cough and were exposed to pertussis by their vaccinated siblings or parents. The incidence of classic symptoms of pertussis decreased with age, and atypical pertussis was usually culture negative but rapidly diagnosed by measurement of the IgM- and IgA-class antibodies by ELISA.