Lower respiratory tract illnesses occurring during the first year of life in 1,179 healthy infants enrolled in the Children's Respiratory Study, Tucson, Arizona, are described. The children, who use the pediatricians of a health maintenance organization, were enrolled into the study between May 1980 and January 1984. Data were collected on signs, symptoms, and diagnosis for each illness; nasopharyngeal and throat swabs were collected at the acute visit for viral, chlamydial, and mycoplasmal cultures. The cumulative incidence of illness in the first year of life was 32.88 per 100 children. Of the 348 initial lower respiratory tract illnesses occurring in these infants, 60% were diagnosed as bronchiolitis. At least one infecting agent was identified in 66% of the specimens collected at the time of the first illness. Respiratory syncytial virus was the most common isolate; 12 other agents were also identified. There was a strong (p less than 0.0001) relation between agent identified, symptoms reported, and diagnosis; bronchiolitis was predominantly associated with respiratory syncytial virus and croup with parainfluenza viruses. Sex and ethnicity were unrelated to illness experience or to characteristics of the first illness. Lower respiratory tract illness occurrence in the Children's Respiratory Study appears to be similar to patterns observed elsewhere, suggesting that diagnoses (and infecting agents) have changed little over the past decades.