Data from an 11-year study of acute lower respiratory tract illness were analyzed to provide added insight into the causes and patterns of occurrence of pneumonia in an outpatient pediatric population. Over 80% of all episodes of pneumonia occurred in children less than seven years of age; the peak attack rate was observed in 2-4-year-old children. A virus or Mycoplasma pneumoniae was isolated from 24% of children with pneumonia; 57% of all episodes occurred during outbreaks for which a viral or mycoplasmal cause could usually be identified. Respiratory syncytial virus (RSV), parainfluenza viruses types 1 and 3, adenoviruses, influenza A viruses and M. pneumoniae accounted for 86% of all isolates. Attack rates of pneumonia in preschool children were highest during periods of RSV and influenza A virus prevalence; M. pneumoniae was the most frequent isolate from school aged children. The data contained in this report augment the understanding of the causes of pneumonia in children and provide a data base for scientists interested in the prevention of lower respiratory tract illness in children.