The mechanism of orange juice-induced heartburn is unclear. One previous uncontrolled study showed only a transient fall in lower esophageal sphincter pressure (LESP) after orange juice and suggested that orange juice-induced heartburn was caused by a direct effect on the esophageal mucosa. We studied the effect of orange juice on LESP by comparing symptomatic patients with asymptomatic controls. LESP was measured for 10 min before and 60 min after the ingestion of 250 ml of orange juice in eight patients with orange juice-induced heartburn and in seven asymptomatic controls. Before orange juice ingestion, LESPs were similar in the symptomatic and asymptomatic groups, 18.3 and 17.8 mm Hg, respectively (not significant). In the control group, LESP increased by at least 5 mm Hg at 20 min after orange juice ingestion and remained elevated for the duration of the observation. In the symptomatic group there was no significant change in LESP despite the onset of heartburn. This was significantly different from the control group (p less than 0.01). We conclude that LESP response to orange juice is different in those who develop heartburn from those who do not. This abnormal response may make these patients more prone to reflux, but the numerical change in LESP is small and the LESP remains within the normal range. We believe that gastroesophageal reflux is unlikely to be the mechanism of orange juice-induced heartburn.