The purpose of this study was to demonstrate that a simple submaximal "step-test" could be used as an exercise challenge to identify elementary school students with suspected but undiagnosed asthma. This article also describes a protocol for exercise testing that can be used in epidemiological evaluations. School age children grades 1-4 with suspected but undiagnosed asthma were identified by a 12-item questionnaire completed by a parent or guardian. Only students identified with suspected asthma by questionnaire were exercise challenged on a step-test if baseline spirometry was normal and there was no contraindication for intense aerobic activity. Possible asthma was defined as a 15% or greater decrease in FEV1 or a 25% or greater decrease in FEF25-75 from baseline at either 3 or 10 minutes. The exercise protocol included spirometry before and after stepping continuously for 5 minutes at an exercise intensity sufficient to maintain a heart rate between 150 and 200 beats per minute. Heart rate was continuously mon-itored throughout the exercise period. Testing was completed at school. No complications occurred during the exercise testing. Exercise testing was completed on 548 students with suspected undiagnosed asthma. Thirty students (6%) had exercise lest changes in pulmonary function that met established criteria for suspecting asthma. A board-certified pediatric allergist/immunologist or private physician examined 26 of the 30 students with positive exercise testing. Asthma was diagnosed in 23 (88.89%) of these students. All students with impaired pulmonary function after exercise were able to return to class after a short period of obsen-ation. In conclusion, a simple, reproducible school-based exercise protocol can be used to identify students with suspected undiagnosed asthma.