We examined the effect of 150 ml of caffeinated instant coffee at two pHs, 4.5 and 7.0, on lower esophageal sphincter pressure in 20 normal volunteers and 16 patients with reflux esophagitis. When ingested alone coffee at pH 4.5 and 7.0 caused a decrease in basal sphincter pressure in normal volunteers from 19.4 +/- 1.5 to 13.7 +/- 1.0 mmHg (P ¿ 0.01) and from 18.7 +/- 1.5 to 16.0 +/- 0.8 mmHg (P < 0.05) respectively. When coffee at pH 4.5 was drunk with a mixed nutrient test meal, the resting sphincter pressure in normal subjects fell after 30-60 min with the nadir, 11.2 +/- 1.0 mmHg, being recorded at 60 min (P < 0.01). Coffee at pH 7.0 with the test meal resulted in a fall in pressure to 14.3 +/- 1.5 mmHg (P < 0.02) at 60 min. In patients with reflux esophagitis, coffee at pH 4.5 lowered lower esophageal sphincter pressure from 9.1 +/- 1.0 to 5.5 +/- 0.6 mmHg (P < 0.005); coffee at pH 7.0 decreased lower esophageal sphincter pressure from 8.5 +/- 1.1 to 6.9 +/- 0.7 mmHg (P < 0.05). In these patients, mean basal pressure, 9.2 +/- 0.8 mmHg, decreased to 5.2 +/- 0.7 mmHg (P < 0.001) 45 min after drinking coffee at pH 4.5 with the test meal. Coffee at the neutral pH caused a fall in pressure from 8.8 +/- 1.1 to 6.5 +/- 0.7 mmHg at 60 min after the test meal. Thus, coffee at either pH 4.5 or 7.0 caused a decrease in fasting and postcibal lower esophageal sphincter pressure in normal volunteers and patients with reflux esophagitis. The magnitude and the duration of the effect were greater after coffee at the lower pH. These data support the clinical belief that coffee may cause or aggravate heartburn by decreasing lower esophageal sphincter pressure.