A double-blind, randomized, placebo-controlled clinical trial was performed in Mexico City to evaluate the efficacy of thalidomide in treating oral recurrent aphthae in human immunodeficiency virus (HIV)-infected subjects. Sixteen HIV-infected patients with clinical and histological diagnosis of oral recurrent aphthous ulcerations received randomly an 8-week course of either thalidomide or placebo, with an initial oral dosage of 400 mg/d for 1 week, followed by 200 mg/d for 7 weeks. Ten subjects received thalidomide and six received placebo. At 8 weeks, nine subjects (90%) in the thalidomide group had complete healing of their ulcers, compared with two (33.3%) of the six patients in the placebo group (P = .03). There was a significant reduction in largest ulcer diameter in the thalidomide group. Rash was observed in 80% of the thalidomide patients. Although thalidomide demonstrated an unquestionable benefit in treatment of oral ulcers in HIV patients, caution must be taken given the frequent occurrence of side effects.