1. Absorption of copper was determined by the simultaneous administration of 64Cu orally and 67Cu intravenously to six patients with Wilson's disease (WD), eighteen of their parents and siblings, four normal subjects and three subjects with cirrhosis of the liver. Absorption was calculated by three methods: (1) the mean ratio of 64Cu to 67Cu body retention at 3 and 4 days as determined by whole-body counting; (2) the mean ratio of 64Cu to 67Cu at 3 and 4 days as determined by faecal excretion; and (3) the mean ratio of 64Cu to 67Cu plasma radioactivity 6–24 h after administration.
2. The total-body counting and faecal methods for determining copper absorption agreed with each other, demonstrating that the normal absorption of copper is 40–70% (mean 56%) of the dose and that absorption is not influenced by cirrhosis of the liver, age or sex; but it appears to be inversely related to the amount of carrier copper. The absorption of copper in both homozygotes and heterozygotes for WD did not differ significantly from that of the control subjects. Therefore, the increased body burden of copper in WD does not appear to be due to over absorption, but rather to decreased biliary excretion of copper.