The underdiagnosis of asthma has not been well studied previously. The purpose of this study was to determine whether the diagnosis of asthma had been made previously in young adults who were subsequently shown to have asthma upon entering military service. Questionnaires were answered by all airmen basic referred to the Allergy-Immunology Clinic at Wilford Hall United States Air Force Medical Center over an 11-month period. There were 192 airmen basic referred to rule out asthma, primarily because of respiratory symptoms with exercise. These airmen were evaluated by history, physical examination, pulmonary function tests and, in 67%, by histamine or exercise challenge test also. In those diagnosed with asthma, the evaluating allergist then determined whether the diagnosis of asthma had been made previously considered or not considered. In airmen basic diagnosed by our clinic with asthma, the diagnosis had been made previously in 45%, considered in 25%, and not considered in 30%. Undiagnosed asthmatic airmen tended to have mild and primarily exercise-induced disease. We conclude that there is a high incidence of underdiagnosis of asthma in the population studied. Exercise-induced asthma accounted for most of the undiagnosed asthma.