Patients with gastroesophageal reflux often describe heartburn after "spicy meals." One ingredient common to most such meals is onion. We investigated the effects of onion on acid reflux and reflux symptoms in 16 normal subjects and 16 heartburn subjects. Subjects were studied with an esophageal pH probe for 2 h after the ingestion of a plain hamburger and a glass of ice water. The identical meal, with the addition of a slice of onion, was ingested on a counterbalanced day. Variables measured were number of reflux episodes, percentage of the time pH was less than four, heartburn episodes, and belches. Ingestion of onions did not increase any of the reflux variables measured in normals. However, onions significantly increased all measures in heartburn subjects, compared with the no-onion condition, and compared with normals under the onion condition. Onions can be a potent and long-lasting refluxogenic agent in heartburn patients.