The efficacy of zinc as a therapeutic agent to control copper balance in Wilson's disease patients has been previously documented with balance studies. In an attempt to develop a simpler and faster tool for evaluating the adequacy of zinc therapy, a technique that measures the uptake into blood of a small oral dose of 64copper was studied in conjunction with copper balance. The mean peak 64copper uptake into blood of nine Wilson's disease patients on D-penicillamine, trien, or no medication was 6.04 +/- 2.74%, comparable with normal controls. Seven patients on zinc therapy had a markedly and significantly reduced mean uptake of 0.79 +/- 1.05% after treatment. The data demonstrate that the prevention of copper uptake into blood in Wilson's disease patients by zinc therapy can be evaluated by 64copper uptake and that peak uptakes of less than 1% occur in patients with neutral or negative copper balance.