BACKGROUND & AIMS: Gastroesophageal reflux can be acid, nonacid, pure liquid, or a mixture of gas and liquid. We investigated the prevalence of acid and nonacid reflux and the air-liquid composition of the refluxate in ambulant healthy subjects and patients with reflux disease (GERD). METHODS: Twenty-four-hour ambulatory recordings were performed in 30 patients with symptomatic GERD and erosive esophagitis and in 28 controls. Esophageal pH and impedance were used to identify acid reflux (pH drop below 4.0), minor acid reflux (pH drop above 4.0), nonacid reflux (pH drop less than 1 unit + liquid reflux in impedance), and gas reflux. RESULTS: The total rate of gastroesophageal reflux episodes was similar in patients and controls. Patients with GERD had a higher proportion (45% vs. 33%) and rate of acid reflux than controls (21.5 [9-35]/24 h vs. 13 [6.5-21]/24 h; P < 0.05). One third of reflux events was nonacid in both groups. Mixed reflux of gas and liquid was the most frequent pattern with gas preceding liquid in 50%-80% of cases. Pure liquid reflux was more often acid in patients with GERD than controls (45% vs. 32%; P < 0.05). CONCLUSIONS: Reflux of gastric contents was similarly frequent in patients with GERD and controls. Although there was no difference in the overall number of reflux episodes, more acidic reflux occurred in symptomatic patients with GERD, suggesting differences in gastric acid secretion or distribution.