To investigate the diagnostic value of 24-hr urinary copper excretion testing after penicillamine challenge in the diagnosis of Wilson's disease, 75 consecutive children referred for a variety of liver problems and in whom parameters of copper metabolism had been investigated were analyzed retrospectively. Seventeen had Wilson's disease, 22 had autoimmune chronic active hepatitis, 6 had primary sclerosing cholangitis, 12 had chronic liver disease of various etiologies, 4 had cryptogenic acute liver failure, 6 had acute hepatitic illnesses and 8 had a variety of disorders featuring normal liver histological appearance. Serum ceruloplasmin and total copper levels were significantly lower in Wilson's disease patients compared with all other groups, but three children with Wilson's disease had normal ceruloplasmin levels and seven had normal total copper levels. No significant difference was found for free serum copper levels and liver copper content between Wilson's disease patients and the other groups. Baseline 24-hr urinary copper excretion was significantly higher in Wilson's disease patients compared with that of the other patients, but six children with Wilson's disease had levels just above the upper limit of normal, overlapping with values obtained in three children with liver failure, two with acute hepatitis, two with autoimmune chronic active hepatitis and three with primary sclerosing cholangitis. The 24-hr urinary copper excretion after penicillamine challenge proved the most accurate single diagnostic test; levels more than 25 mumol/24 hr were present in 15 of 17 patients with Wilson's disease, but in only 1 child with liver failure of the 58 with other disorders.(ABSTRACT TRUNCATED AT 250 WORDS)