The effect of acid application in the esophagus on pulmonary function was measured in three groups of patients in a double-blind investigation. Group 1 consisted of ten patients with esophagitis but without pulmonary disease, group 2 was 21 patients with bronchial asthma without esophagitis, and group 3 was eight patients with both esophagitis and bronchial asthma. Pulmonary function was assessed by total lung capacity (TLC), airway resistance (Raw), residual volume (RV), and peak expiratory flow (PEF). The four parameters were measured at the following five times during a single trial: before and after insertion of an esophageal catheter; after instillation of 50 ml of isotonic sodium chloride solution; after instillation of 50 ml of 0.1 N hydrochloric acid; and after intravenous injection of atropine (0.01 mg/kg of body weight). A significant decrease (the Wilcoxon test, p less than 0.02; and the Mann-Whitney test, p less than 0.002) in PEF and a significant increase (p less than 0.02 and p less than 0.002, respectively) in Raw after instillation of HCl were seen only in group 3. Changes in the other groups were small and without any regular pattern. Six of the patients in the third group accepted another trial after three days of pretreatment with atropine (0.01 mg/kg twice daily). Now instillation of acid did not produce any change in PEF or Raw (p less than 0.001). We conclude that a modest bronchoconstriction when acid is present in the esophagus is seen in patients with bronchial asthma and severe esophagitis. Atropine inhibits this bronchoconstriction, indicating vagal mediation.