The clinical course, results of standard laboratory tests, parameters of copper metabolism, and hepatic morphology in 9 cases (3 of our own and 6 from the literature) of Wilson's disease presenting as fulminant hepatic failure were compared with the findings in 5 cases of idiopathic fulminant hepatic failure. Patients with Wilson's disease were usually younger, and 7 of the 9 patients had Kayser-Fleischer rings. Patients with idiopathic fulminant hepatic failure had elevated 24-h urinary copper, decreased ceruloplasmin, and low or normal serum copper. Fulminant hepatic failure with Wilson's disease differed from idiopathic fulminant hepatic failure by the following biochemical findings: (a) higher copper levels in serum, urine and liver; (b) less pronounced elevations of transaminase levels; (c) higher concentrations of total bilirubin; and (d) lower hemoglobin values. Serum copper was the most useful biochemical test in diagnosing Wilson's disease before death. At autopsy, only hepatic copper concentrations clearly separated the two groups. Serial serum copper levels (antemortem) and quantitative analysis of hepatic copper (after recovery or postmortem) in patients with fulminant hepatic failure should help to exclude Wilson's disease.