ABSTRACT Twenty-four hour monitoring provides a continuous record of the pH of the lower esophagus in a near physiologic setting. The upper level of physiologic reflux was determined from the percentage of time and the number of episodes that the pH was less than 4 or more than 7 and the mean duration of each episode in fifteen asymptomatic subjects. One hundred patients with symptoms of gastroesophageal reflux were divided into four groups on the basis of twenty-four hour pH monitoring: those with abnormal acid but normal alkaline reflux, termed acid refluxers (51 patients); those with both abnormal acid and alkaline reflux, termed acid-alkaline refluxers (25); those with normal acid and abnormal alkaline reflux, termed alkaline refluxers (6); and those with both normal acid and alkaline reflux, termed nonrefluxers (18).Nonrefluxers had a similar incidence of heartburn, regurgitation, and dysphagia as acid and acid-alkaline refluxers, proving the inaccuracy of symptoms for detecting reflux. Alkaline refluxers had a lesser incidence of heartburn but a greater incidence of regurgitation, and four alkaline reflux patients presented with severe pulmonary disease secondary to aspiration. Similar incidence and degree of esophagitis was seen in acid, acid-alkaline, and alkaline refluxers. All three groups of symptomatic refluxers had a mean distal esophageal sphincter pressure significantly lower than that of the control asymptomatic subjects. There was no difference in the distal esophageal sphincter pressure between controls and symptomatic nonrefluxers.Nine of the patients with acid-alkaline reflux and one of the patients with alkaline reflux underwent an antireflux procedure and were restudied three months postoperatively. All ten patients had a 24 hour pH acid score within normal limits, but two had an abnormal 24 hour pH alkaline score. In both patients, reflux was demonstrated after placing an acid load in the stomach.It is concluded that symptomatic gastroesophageal reflux in patients with an intact gastrointestinal tract is a mixture of both acid and alkaline secretions, with one or both abnormal due to different degrees of acid production and pyloric regurgitation. Patients with alkaline reflux may develop serious complications of reflux in the absence of typical symptoms of heartburn. Twenty-four hour pH monitoring of the esophagus is useful in the identification of these patients and in evaluating the ability of an antireflux procedure to control both abnormal acid and alkaline reflux.