The Tucson Children's Respiratory Study, Tucson, Arizona, has been established as a long-term, longitudinal, prospective study of the risk factors for acute lower respiratory tract illnesses in early childhood and for chronic obstructive airways disease in later life. A total of 1,246 newborns were enrolled into the study between May 1980 and January 1984, representing 78% of eligible infants. Cord blood for immunologic studies, neonatal blood specimens for blood counts and differentials, and blood specimens at nine to 15 months of age for immunologic studies, blood counts, and differentials have been obtained on the majority of enrolled children. Pre-illness physiologic and more detailed immunologic studies have also been done on large subgroups of subjects. The majority of lower respiratory tract illnesses suffered by these children in the first three years of life have been assessed in detail for etiologic agents by means of culture and serologic techniques; 1,052 illnesses have been evaluated thus far. The type of illness and nature of etiologic agents are very similar to those reported in other epidemiologic studies. Thus, this group of enrolled infants and their family members constitute an appropriate population for the long-term study of risk factors for acute and chronic respiratory disorders.