One hundred and twenty-four patients who had had recurrent aphthous stomatitis (RAS) for two to 53 years took part in three studies to assess the effectiveness of levamisole and determine an adequate regimen. In study 1 and the first half of study 2 both of which were double-blind, levamisole 150 mg/day (or placebo) was given on three consecutive days in every fortnight. In the second two months of study 2 and in the open trial (study 3) three-day courses were given only when an episode of RAS occurred. The drug was well tolerated. The signs and symptoms of RAS improved gradually and significantly in those treated with levamisole but not in those on placebo, and intergroup differences were also significantly in favour of the active drug. Improvement occurred earlier in study 3 than in the other two studies. Hence levamisole may prevent new episodes of RAS.