A multicentric crossover randomized trial of 100 mg of thalidomide vs placebo each for 2 months was conducted in patients with severe aphthous stomatitis of more than 6 months' duration. Seventy-three patients were included. Complete remission was obtained in 32 patients who received thalidomide and in 6 patients who received placebo. The confidence interval of the difference between the two treatments ranged from 25% to 53%. Most of the patients who did not achieve a complete remission had a dramatic improvement with regard to the number of aphthae when they were receiving thalidomide. Thirteen of 17 patients who had a complete remission while they were receiving thalidomide had a recurrence with placebo, 19 +/- 9 (mean +/- SD) days after stopping this drug. Side effects were significantly more frequent with thalidomide, especially drowsiness and constipation. We concluded that thalidomide in a dosage of 100 mg/d is an effective treatment of severe aphthous stomatitis but is not without some risk.