Twenty-three patients with Wilson's disease (WD) treated with D-penicillamine underwent clinical examination, as well as laboratory and motor testing. The clinical findings were scored. Laboratory tests included determination of the caeruloplasmin level, the free serum copper level, 24 h urinary copper excretion, liver enzymes and in 10 patients liver copper content of a liver biopsy. Laboratory tests and clinical scores were correlated. To quantify impairment of voluntary movements in WD fastest possible isometric index finger extensions and fastest alternating finger movements were analysed. Eleven patients presented with abnormally slow and 15 with abnormally irregular voluntary movements. Slowness of alternating movements correlated with the clinical score. The clinical score also correlated with the duration of symptoms prior to onset of therapy. Motor testing turned out to be sensitive enough to monitor improvement of neurological symptoms after onset of therapy. Comparison with motor testing in other basal ganglia diseases and cerebellar patients showed differences to patients with Parkinson's and Huntington's disease and similarities to patients suffering from AIDS-related dementia. In a small number of WD-patients similar results as in patients with a degenerative cerebellar disease were found.