Recurrent oral and genital ulcers are the most frequent problem in the management of the Behçet syndrome. Uncontrolled experience suggests that thalidomide may help prevent recurrences of these ulcers. To determine the efficacy of two thalidomide dosages in the treatment of mucocutaneous lesions of the Behçet syndrome. Randomized, double-blind, placebo-controlled trial. Specialist outpatient clinic for the Behçet syndrome in Turkey. 96 male patients with the Behçet syndrome who primarily had mucocutaneous lesions without major organ involvement. Thalidomide, 100 mg/d or 300 mg/d, or placebo for 24 weeks. Sustained absence of any oral and genital ulceration during treatment (complete response) and changes in the number of mucocutaneous lesions. An additional evaluation was done 4 weeks after treatment ended. A complete response occurred in 2 of the 32 patients (6% [95% CI, 0.8% to 20.8%]) receiving thalidomide, 100 mg/d; in 5 of the 31 patients (16% [CI, 5.5% to 33.7%]) receiving thalidomide, 300 mg/d; and in none of the 32 patients (0% [CI, 0% to 10.9%]) receiving placebo (P = 0.031). The suppressive effect of thalidomide with either dosage was evident at 4 weeks for oral ulcers (P < 0.001) and at 8 weeks for genital ulcers (P < 0.001) and follicular lesions (P = 0.008). This effect persisted during treatment but diminished rapidly after treatment was discontinued. Both thalidomide dosages led to significant increases in the number of erythema nodosum lesions during the first 8 weeks of treatment (P = 0.03). Polyneuropathy developed in 4 patients (1 in the 100-mg/d group and 3 in the 300-mg/d group); in 3 of these patients, the condition was diagnosed after the trial had ended. Thalidomide is effective for treating the oral and genital ulcers and follicular lesions of the Behçet syndrome. A dosage of 100 mg/d is as effective as a dosage of 300 mg/day.