The reasons why few patients with gastro-oesophageal reflux disease develop oesophagitis are not yet clear. One of the factors whose role is still debatable is the gastric acid secretory state. The aim of this study was to evaluate whether differences exist in nocturnal gastric acidity between patients with oesophagitis and refluxers without oesophageal lesions. We studied 65 patients with gastro-oesophageal reflux disease, 37 of whom presented erosive oesophagitis, while 28 had no oesophageal lesions. Thirty-one healthy volunteers were used as controls. In both patients and controls intragastric and intraoesophageal pH were measured continuously using 2 in-dwelling glass electrodes, placed in the gastric corpus and in the oesophagus. Mean intragastric pH was calculated over 3 nocturnal time periods: 11.00 p.m.-07.00 a.m.; 11.00 p.m.-03.00 a.m.; 03.00 a.m.-07.00 a.m. Patients with oesophagitis had a lower nocturnal gastric pH (1.6 +/- 0.2) than either refluxers without oesophagitis (2.2 +/- 0.3) (p = 0.05) or controls (2.6 +/- 0.4) (p = 0.02). The difference occurred entirely in the second part of the night. Furthermore, in the same time period, oesophagitis sufferers had a higher percentage of oesophageal acid exposure at pH < 2 (0.7 +/- 0.2) than refluxers without oesophagitis (0.2 +/- 0.1) (p = 0.05), suggesting that gastric findings are of pathogenetic relevance. Patients with reflux oesophagitis have a higher nocturnal intragastric acidity than refluxers without oesophagitis. This difference, confined to the second half of the night, may be due to an altered circadian pattern of gastric acid secretion and may partially explain why only some refluxers develop oesophagitis.