The 1977 Montana legislature funded an extensive Montana Air Pollution Study (MAPS). One hundred and twenty children in the fourth and fifth grades in Missoula, Montana had their pulmonary functions tested on 6 days during the 1979-80 school year. On May 18, 1980, Mount St. Helens erupted, resulting in very high total suspended particulates (TSP) levels due to ash (peak day = 11,054 micrograms/m3 24-hour average) through May 22. When these children returned to school on May 23, they had their pulmonary functions tested a seventh time. There was no substantial decrease in pulmonary function after the ash exposure. By comparison, the childrens' pulmonary tests did show a significant decrease after 3 days of high urban air pollution (440 micrograms/m3 3-day average). Businesses and schools were closed for 4 days after the eruption and people were advised to remain indoors; hence, lack of measured pulmonary function test effects from the ash may be due to the protective precautions taken by the children.