Analysis will be stratified for Crohn's disease and ulcerative colitis. Given the hypotheses that thalidomide will be effective in inducing clinical remission at 8 weeks in 60% of patients with Crohn's disease, and 55% of patients with ulcerative colitis, compared to 20% of patients with placebo, with a significance of 0.05% and a power of 80%, estimated needed sample size is of 124 patients.