Aspiration of gastric acid into the trachea may cause asthma in some patients who have gastroesophageal reflux. Antireflux surgery has been advocated for such patients, but lack of an objective test for acid aspiration makes patient selection difficult. We report a new technique for demonstrating acid aspiration, simultaneous tracheal and esophageal pH monitoring. Tracheal pH was measured with a 1.0-mm pH electrode introduced through the cricothyroid membrane under bronchoscopic vision. A standard esophageal pH electrode was placed in the usual position. Tracheal and esophageal pH were monitored over a 24-hour period. Peak expiratory flow rate was measured hourly while the patient was awake. We present data obtained in 3 patients with severe asthma and symptomatic gastroesophageal reflux. All 3 patients demonstrated a decrease in tracheal pH to less than 5.5, coinciding with a decrease in esophageal pH to less than 4.0. The test was repeated after antireflux operation and showed that significant decreases in esophageal pH no longer lowered tracheal pH. Asthmatic symptoms were improved, and medication was reduced in 2 of the 3 patients.