The relationship between gastroesophageal reflux and asthma has not been clearly defined. We measured the lower esophageal sphincter pressures and studied gastroesophageal reflux patterns over 24 hours using an ambulatory Gastroreflux Recorder (Del Mar Avionics, Irvine, CA) in 44 controls and 104 consecutive adult asthmatics. The presence or absence of reflux symptoms was not used as a selection criterion for asthmatics. All asthmatics had discrete episodes of diffuse wheezing and documented reversible airway obstruction of at least 20%. Patients underwent reflux testing while receiving, if any, their usual asthmatic medications: 71.2% required chronic bronchodilators and 28.8% required no bronchodilators. Compared with controls, asthmatics had significantly decreased lower esophageal sphincter pressures, greater esophageal acid exposure times, more frequent reflux episodes, and longer clearance times in both the upright and supine positions (P less than 0.0001 for all parameters tested). There were no differences in any of the measured reflux parameters between asthmatics who required bronchodilators and those who did not. Thus, the decreased lower esophageal sphincter pressures and increased levels of acid reflux in asthmatics were not entirely caused by the effects of bronchodilator therapy. Receiver-operating characteristic analysis generated reflux values that discriminated asthmatics from controls. More than 80% of adult asthmatics have abnormal gastroesophageal reflux. We conclude that most adult asthmatics, regardless of the use of bronchodilator therapy, have abnormal gastroesophageal reflux manifested by increased reflux frequency, delayed acid clearance during the day and night, and diminished lower esophageal sphincter pressures.