From January 1986 to December 1987, 596 children less than 5 years of age with lower respiratory tract infection (LRI)--manifested as laryngitis, croup, bronchitis, bronchiolitis, and pneumonia--were studied for evidence of infection with respiratory tract viruses Mycoplasma pneumoniae, and Chlamydia trachomatis. Of the 596 children in the study, 315 were ambulatory and 281 were hospitalized. Virologic studies included isolation and rapid diagnosis of virus from specimens of nasopharyngeal aspirate (NPA) and serologic studies of blood samples. Cultures of NPA for C. trachomatis were performed for children less than 6 months of age who had pneumonia. Of the LRI cases, 45% were associated with viral infections of the respiratory tract and 12.1% were associated with C. trachomatis. Respiratory syncytial virus (RSV) accounted for 45.2% of infections with viral agents and was associated with acute bronchitis, acute bronchiolitis, and pneumonia. Parainfluenza type 3 virus was the most common virus found in conjunction with laryngitis and croup. The incidence of infections due to RSV peaked in July and August, while that of infections due to parainfluenza viruses peaked in February and March; influenza viruses and adenoviruses were isolated throughout the year.