Study objective: To evaluate the role of microaspiration in gastroesophageal reflux-induced bronchoconstriction. Design: Prospective study blinded to the subject. Setting: Outpatient laboratory of a 908-bed university hospital. Participants: Thirty nonsmoking adults divided into two groups: asthmatics with reflux (AR), 20; and subjects with gastroesophageal reflux (R), 10, interventions: Dual esophageal pH probe placed. Esophageal infusions of normal saline solution, 0.1 N hydrochloric acid, then normal saline solution, each lasting 18 min, were followed by two 20-min recovery periods. Subjects remained in the supine position throughout. Spirometry and specific airway resistance (SRaw) performed at baseline, after each esophageal infusion and recovery period. Proximal esophageal acid exposure, a requirement for microaspiration, was assessed by the proximal esophageal pH probe. Results: Peak expiratory flow rate (PEF) decreased with esophageal acid in the AR group and did not recover immediately despite esophageal acid clearance with a significant main effect of subject groups (p<0.021) by repeated measures analysis of covariance. This decrease in PEF was not associated with the presence of proximal esophageal acid exposure (p=0.618). Specific airway resistance increased in the AR group with esophageal acid and worsened despite acid clearance, especially during the second recovery phase, with a significant phase (p<0.009) and group by treatment effect (p<0.009). The presence of proximal esophageal acid exposure was not associated with this deterioration in SRaw (p= 1.0). Conclusions: Esophageal acid infusions given in the supine position caused a decrease in PEF and an increase in SRaw in the asthma with reflux group, which did not improve despite acid clearance. These responses were not dependent on proximal esophageal acid exposure. Also, SRaw continued to worsen during the recovery phase in the AR group, which may represent a delayed bronchoconstrictor effect. These data suggest that microaspiration does not play a significant role in esophageal acid-induced bronchoconstriction.