Whereas human respiratory effects of brief ozone exposures are well documented, much less is known about the human health effects of mid- to long-term exposures. The authors' objective in this study was to determine whether lung function or respiratory symptom changes would occur over the course of a summer season among healthy young adults working outdoors in the presence of ozone. The authors studied 72 sophomore cadets from the U.S. Military Academy at West Point, New York, 21 of whom attended special summer training in Fort Dix, New Jersey, an area characterized by elevated ozone levels; the remaining cadets attended training in areas with moderate ozone levels (i.e., Fort Benning, Georgia; Fort Leonard Wood, Missouri; and Fort Sill, Oklahoma). The authors hypothesized that adverse respiratory outcomes, if any, would be more pronounced in the group exposed to higher ozone levels. Spirometry was performed and respiratory symptoms were assessed-both before and after the summer-in a clinic at West Point. Time spent outdoors during summer training averaged 11 hr/d. Both mean and peak ozone levels were higher at Fort Dix than at the three remaining sites. Regional levels of sulfur dioxide and particulate matter less than 10 microm in aerodynamic diameter were relatively low during the study. However, all cadets reported frequent exposure to dust, exhaust, and smoke in the course of their training. Averaged across all subjects, there was a statistically significant drop in forced expiratory volume in 1 sec of 44 ml (p = .035) over the summer. There were also significant increases in reports of cough, chest tightness, and sore throat at the follow-up clinic visit. A larger mean forced expiratory volume in 1 sec decline was observed at Fort Dix, where ozone exposures were the highest. The results of this study demonstrated a seasonal decline in respiratory function among healthy young adults working outdoors in the presence of ozone and particulate matter.