Prolonged ambulatory pH monitoring was performed on 89 patients with previous diagnosis of asthma (27 patients), chronic cough (28 patients), noncardiac chest pain (34 patients), and on 27 healthy control subjects. The extent of gastroesophageal reflux (GER) was determined using a catheter containing two antimony pH electrodes positioned 5 cm and 20 cm above the superior border of the manometrically determined lower esophageal sphincter. Reflux was defined as a drop in pH to < 4 in the distal esophagus. We compared both pH < 4 and pH < 5 as the beginning of reflux episodes for the proximal esophagus. Considering the confidence interval of 95% in healthy control subjects as a normality criterion, we found a prevalence of abnormal distal GER in 44% of asthmatics, 50% of patients with cough, and 53.8% of patients with noncardiac chest pain. Abnormal proximal acid exposure was found in 24% of asthmatics, 10.7% of patients with cough and 44.1% of patients with chest pain. Distal acid exposure was significantly longer than proximal esophageal acid exposure in all patient groups (p < 0.05). There were no differences in the evaluation of proximal GER comparing pH < 4 with pH < 5. The data also indicate a tendency toward upright, rather than supine acid exposure. These results support the use of 24-h pH monitoring in patients with chest complaints and indicate that GER may frequently be involved in the pathogenesis. They do not support the theory that proximal GER is a specific etiologic factor in chronic cough or asthma.