A retrospective study relating air pollution levels to hospital admissions for acute exacerbations among adults with chronic respiratory illness and children with acute respiratory disease was carried out in a highly industralized Canadian city. A total of 1,780 patient records were collated, and daily, weekly, and monthly hospital admission rates were related to a standard air pollution index that took into account floating particles and sulfur dioxide. A correlation coefficient of 0.77 (P less than 0.01) between number of admissions and level of air pollution was found for the city as a whole, and the strength of the association varied inversely with the distance of the hospital from the industrial area.