A competitive relationship exists between copper and zinc: among other effects, excessive dietary zinc is known to decrease the absorption of copper from the gastro-intestinal tract. The purpose of this study is to investigate the effectiveness of oral zinc therapy in 4 patients with Wilson's disease, who, during a 3-year period, took zinc as their only medication to influence their copper metabolism. Physical examinations, oral 64Cu loading tests, plasma concentrations of copper, zinc and ceruloplasmin, and the urinary copper excretion were used to monitor the effect of therapy. The dosages used ranged from 3 x 100 to 3 x 400 mg zinc sulphate per day. The clinical and biochemical results of the oral zinc therapy were good in all 4 patients and no toxic side-effects were seen. Our conclusion from this study is that oral zinc may well be a low toxic alternative to D-penicillamine in the treatment of Wilson's disease.