A multicenter, double-blind, randomized, placebo-controlled study was conducted to determine the safety and efficacy of thalidomide in reduced, intermittent doses for preventing recurrences of oral and esophageal aphthous ulcers in patients with human immunodeficiency virus (HIV) infection. Forty-nine HIV-infected patients whose ulcers previously had healed as a result of thalidomide therapy were randomly assigned to receive either 100 mg of oral thalidomide or placebo 3 times per week for 6 months. Ulcers recurred in 14 (61%) of 23 thalidomide-randomized patients, compared with 11 (42%) of 26 placebo-randomized patients, with no significant difference in the median time to recurrence of ulcers (P=.221). There were no changes in plasma levels of HIV RNA, tumor necrosis factor (TNF)-alpha, and soluble TNF receptor II at the time of ulcer recurrence. Adverse events among patients treated with thalidomide included neutropenia (5 patients), rash (5 patients), and peripheral sensory neuropathy (3 patients). Thalidomide in lower intermittent doses is ineffective at preventing recurrence of aphthous ulcers in HIV-infected persons.