In order to study the acute effects of environmental tobacco smoke on lung function, a cigarette sidestream smoke provocation test (analogous to a nonspecific bronchial provocation test) was performed. Ten persons with airways hyperreactive to methacholine and ten normoreactive persons were tested. A dose-response relationship was found for symptoms. The lung function of the normoreactive persons was not altered by short-time inhalation of sidestream smoke. The hyperreactive subjects, however, experienced significant decreases in FEV1, FVC, and MEF50. The decrease was most pronounced after the lowest dose of 2 ppm smoke-induced CO, leading to a mean fall of 6.3 percent in FEV1. Five of ten subjects with hyperreactive airways showed a decrease in FEV1 of more than 10 percent during the sidestream smoke provocation, one of them a decrease of over 20 percent after inhalation of 16 ppm CO sidestream smoke. We conclude that even short exposure to low concentrations of cigarette sidestream smoke causes significant impairment of lung function in sensitive persons.