The biliary excretion of copper was measured in eight patients with Wilson's disease (three untreated, with hepatic dysfunction) and 10 control subjects (three with hepatic dysfunction). The duodenum was perfused with an amino-acid solution containing a non-absorbed marker, ⁵¹CrCl₃, and juice was aspirated from the duodeno-jejunal junction. The mean concentration of copper in the duodenal aspirate in Wilson's disease was significantly lower than in the control group. The mean biliary copper excretion rate in Wilson's disease of 8·6 ± 0·8 μg/20 min (SEM) was also significantly below that of the control group (16·4 ± 0·8 μg/20 min). The presence of liver dysfunction made no significant difference to the excretion rates in either group of patients. These results suggest that the copper accumulation in Wilson's disease is due to the inability of the liver to excrete copper into bile in adequate amounts.