Ambulatory 24-hour esophageal pH monitoring and esophagogastroduodenoscopy were performed in 72 patients with symptoms suggestive of gastroesophageal reflux. Additionally, 22 asymptomatic healthy volunteers underwent pH monitoring. In patients with classic reflux symptoms and endoscopic esophagitis, a mean of 5.41 minutes/hour of reflux below pH 4 was found compared to 0.70 minutes/hour in controls (p less than 0.0001). The mean number and duration of reflux events in this group were 1.51 events/hour and 4.0 minutes/event, compared with 0.31 events/hour and 2.26 minutes/event in volunteers (p less than 0.001, p less than 0.01). A new system for ambulatory esophageal pH monitoring is presented using a pH-sensitive radiotelemetry pill or a pH probe and computerized methods for ambulatory data collection, analysis, and storage. An overall sensitivity of 76% was obtained with a 91% selectivity for detection of acid reflux in 51 patients having classic symptoms of gastroesophageal reflux. Ambulatory pH monitoring was positive for acid reflux in seven of 11 patients with normal endoscopic findings. Conversely, eight of 12 patients with normal pH monitoring had endoscopic esophagitis. In 19 patients presenting with atypical symptoms or previous gastric surgery, endoscopic findings were normal in 15. Nine of these 15 were identified as acid refluxers by pH monitoring. A combined approach using both pH monitoring and endoscopy is warranted for maximal detection and quantification of disease. A clear clinical role for pH monitoring is seen in the early diagnosis of acid reflux, particularly in patients having normal endoscopic findings with nonspecific gastrointestinal complaints or previous gastric operations.