Clinical asthma is estimated to occur in 4 to 7 percent of the US population, but respiratory symptoms are much more common. The correlation between asthma, tests of bronchial hyperresponsiveness, and symptoms is imprecise and difficult to ascertain. The prevalence of exercise-induced airflow obstruction is not well known in healthy adults. We studied 100 consecutive US Air Force members to determine the prevalence of respiratory symptoms and the incidence of airflow obstruction after standardized free- run exercise. Subjects who developed airflow obstruction with exercise underwent methacholine challenge. We found that 31 percent of our population had respiratory complaints, but only 6 percent had significant airflow obstruction after exercise. Three of six subjects with abnormal results on exercise spirometry had bronchial hyperresponsiveness in response to methacholine. In summary, objective evidence of exercise-induced airflow obstruction was uncommon in our healthy subjects, although nonspecific respiratory complaints were frequent. In our study, methacholine challenge was positive in only half the subjects with abnormal spirometric data after exercise. We conclude that the diagnosis of exercise-induced asthma in healthy individuals is difficult because of the uncertain relationship between symptoms and objective testing and between modes of objective testing. Additionally, the role of free-run exercise testing needs further investigation in adults being screened for asthma.