The aim of this work was to evaluate whether oral supplementation with zinc sulphate (ZnSO4) could restore thymic endocrine function in patients with Crohn's disease who showed decreased plasma concentrations of zinc and active thymulin, a zinc-dependent thymic hormone. Twenty-seven patients in clinical remission were randomly assigned to receive, for 3 months, one of the following treatments: 60 mg/day ZnSO4; 200 mg/day ZnSO4 or placebo. Plasma thymulin activity and zinc concentrations significantly increased only in patients treated with 200 mg/day ZnSO4. Lymphocyte subpopulations, within the range of normality before zinc supplementation, were unaffected by any of the administered treatments. In conclusion, low plasma concentrations of zinc and thymulin in Crohn's disease patients were restored by the administration of high doses of zinc.