Foy, H. M. (School of Public Health and Community Medicine, Univ. of Washington, Seattle, Washington 98195), M. K. Cooney, A. J. Maletzky and J. T. Grayston. Incidence and etiology of pneumonia, croup and bronchiolitis in preschool children belonging to a prepaid medical care group over a four-year period. Am J Epidemiol 97: 80–92, 1973.—The epidemiology of pneumonia, croup and bronchiolitis among children less than 6 years old belonging to the Group Health Cooperative of Puget Sound, Seattle, was studied from December 1, 1966 through March 31, 1971. The annual incidence per 1000 was 30 for pneumonia, 2 for bronchiolitis and 7 for croup. The highest rates of pneumonia and bronchiolitis coincided with yearly epidemics of respiratory syncytial virus in winter-spring. A virus or Mycoplasma pneumoniae was isolated from the throat swabs of 17% of pneumonia, 24% of bronchiolitis and 29% of croup cases. Serologic diagnosis was obtained in 37, 36 and 39% of these disease categories, respectively. Although isolates from patients in each disease category included several different agents, respiratory syncytial virus was the most important agent in young infants with pneumonia and bronchiolitis; parainfluenza viruses, especially type 1, dominated in croup; and M. pneumoniae was the most important agent in pneumonia of older children. Since hospitalization rates fell with increasing age, agents recognized in hospitalized patients reflected those affecting the very young, particularly respiratory syncytial virus infection, whereas older patients, who frequently had M. pneumoniae infection, were treated as outpatients. Rhino-, echo- and coxsackie B viruses were isolated, but comparison with a control group indicates that they were probably not the cause of lower respiratory disease.