The paper reports results from an ongoing study of outdoor air pollution and respiratory health status of children living in six cities in the eastern and midwestern United States. The study enrolled 10,106 white preadolescent children between 1974 and 1977 in three successive annual visits to each city. Each child received a spirometric examination and a parent completed a standard questionnaire. Of the cohort, 8,380 children were seen for a second examination one year later. An air pollution monitoring program was begun in each community at about the time of the first examination. For the report, measurements of Total Suspended Particulates (TSP), the sulfate fraction of TSP (TSO4), and sulfur dioxide (SO2) concentrations at study-affiliated outdoor stations were combined with measurements at other public and private monitoring sites to create a record of TSP and SO2 concentrations in each of 9 air pollution regions during the one-year period preceding each examination and, for TSP, during each child's lifetime up to the time of testing.