In the period April to October, 1977, an epidemic of pertussis in the outpatient population of a large metropolitan hospital involved 115 cases that were diagnosed by culture or direct fluorescent antibody tests. A study of secondary cases in household contacts showed attack rates of 81% in children under one year of age; attack rates decreased with increasing age to 8% in persons over 20 years of age. Vaccine efficacy was estimated to be 63%. There was no evidence of decreased efficacy with increasing time after vaccination. Fourteen asymptomatic FA-positive individuals were identified; four of these were also culture positive. Four were adults and ten were children. Nine of the ten children had received three or more vaccinations, compared to only 29 of 78 symptomatic children (P = 0.002).