Ten patients with bronchial asthma and ten control subjects were exposed to sidestream cigarette smoke (passive smoking) for one hour in an environmental chamber. All subjects showed the same increase in carboxyhemoglobin as a result of the exposure: 0.40 percent. The asthmatic group demonstrated a significant linear decrease in pulmonary function during this exposure. After one hour of smoke, FEV1 decreased 21.4 percent, FEF25-75% decreased 19.2 percent, and FVC decreased 20.0 percent in the asthmatic patients. These alterations were readily reversible in all subjects when given inhalations of metaproterenol following the exposure. The control subjects showed no change in pulmonary function when exposed to identical conditions. These data show that nonsmokers with bronchial asthma are at risk when exposed to sidestream cigarette smoke in an environmental chamber.