Marfan syndrome is known to have pulmonary manifestations such as pneumothorax. There have been few previous studies of pulmonary function tests and none of bronchial hyperreactivity. Therefore, pulmonary function tests were performed in 11 children with Marfan syndrome and 11 normal children. Bronchial responsiveness was tested in ten of the Marfan patients by methacholine challenge test and response to bronchodilator. Because of disproportionate length of legs in Marfan patients, an "ideal" standing height was calculated from sitting height. Pulmonary function tests, as absolute values or as percent predicted based on "ideal" height, were not different in Marfan patients and normals, although a few individual patients had abnormal function (mostly airway obstruction and hyperinflation). Response to methacholine challenge was positive on forced expiratory volume in 1 second (FEV1), forced expiratory flow between 25 and 75% VC (FEF25-75%), and FEF50%, in 37.5%, 60%, and 70% of tests respectively. A significant response to bronchodilators was obtained in 40% of patients as measured by FEV1, in 90% by FEF25-75% and in 100% by FEF50%. Pulmonary function tests after bronchodilator were significantly higher when compared with values before the bronchodilator as well as with the baseline before methacholine. Therefore, most if not all patients with Marfan syndrome had hyperreactive airways in this relatively small group of patients. Even though only one patient had a diagnosis of asthma, six more had subtle symptoms. It is concluded that tests for bronchial hyperreactivity could be part of the routine investigation in Marfan syndrome. Further studies on larger numbers of patients are still needed.