A double-blind clinical trial was carried out on thirty-eight patients with symptomatic and endoscopically proven duodenal ulcer to compare the therapeutic responses to mastic (1 g daily, twenty patients) and placebo (lactose, 1 g daily, eighteen patients) given orally over a period of 2 weeks. Symptomatic relief was obtained in sixteen (80%) patients on mastic and in nine (50%) patients on placebo, while endoscopically proven healing occurred in fourteen (70%) patients on mastic and four (22%) patients on placebo. The differences between treatments were highly significant (P less than 0.01). Mastic was well tolerated and did not produce side effects. It is concluded that mastic has an ulcer healing effect, but further studies are needed to establish its role in treating peptic ulcer.