A Controlled, double-blind study was performed to compare the effects of levamisole and placebo in eighteen patients with recurrent aphthous stomatitis. Levamisole is a drug which enhances defective cellular immune responses. It was administered in a flexible dosage schedule: 50 mg. three times daily for three consecutive days at the start of an aphthous lesion but with an interval of at least 2 weeks between courses of therapy. Placebo consisted of pills that were identical in appearance, taken according to the same schedule. Statistical evaluation showed decreased frequency of lesions, shorter duration, and diminished pain of lesions in the group receiving levamisole. Subsequent follow-up in an open trial confirmed these results.