Although administration of zinc to human subjects has been reported to interfere with lymphocyte function, this single report has never been confirmed or refuted. We have developed zinc as a lifelong therapy for patients with Wilson's disease. Interference with lymphocyte function occurring as a side effect of zinc therapy could produce serious problems in our patients. We evaluated lymphocyte mitogenic response and natural killer cell activity in patients with Wilson's disease treated for 5 years or longer with zinc, in comparison with normal controls, and found no differences. In a second study, we evaluated these same parameters in patients with Wilson's disease before and after 1 year of zinc therapy, and again found no significant differences. We have seen no indications of immune suppression or increased susceptibility to infections in our patients, who have now been treated with zinc for up to 15 years. We conclude that any side effects from compromised lymphocyte function caused by administration of zinc are not of concern to patients with Wilson's disease.