The study built on a previous study funded by the Health Effects Institute, a non-profit foundation jointly supported by the U.S. Environmental Protection Agency and vehicle manufacturers. The previous study was a prospective cohort study of nitrogen dioxide (NO2) exposure and respiratory symptoms and illnesses during the first 18 months of life. Between January, 1988 and June, 1990, 1,315 infants were enrolled and then followed with collection of daily respiratory symptom data and serial monitoring of NO2 concentrations in their homes. The full protocol was completed by 823 subjects and a total follow-up experience of over one-half million days was accumulated. Follow-up ended in December, 1991 and analyses related to the study's principal focus, the health effects of NO2, was completed during the fall of 1992.