Patients with Wilson's disease who present with acute neurological symptoms often become clinically worse when initially treated with penicillamine. Other available anticopper drug therapies do not appear to offer a solution to this treatment problem. We are developing and evaluating a new drug, ammonium tetrathiomolybdate for this purpose. Theoretically, tetrathiomolybdate has optimal properties, including an immediate blockade of copper absorption and the property of forming complexes with copper in the blood, rendering the copper nontoxic. In this article, we present results from six patients treated with tetrathiomolybdate for up to 8 weeks as initial therapy. None of the five patients who had presented with acute neurological symptoms worsened. Also presented are methods of assay, preliminary stability studies, and methods of evaluating therapeutic end points with respect to copper metabolism.