Wilson disease is a rare autosomal recessive disease of copper metabolism. The gene for Wilson disease was characterized recently and has been predicted to encode a copper-transporting ATPase highly homologous to the protein encoded by the gene of Menkes disease. In this study, the genetic mutations of two Finnish patients with Wilson disease were investigated. One patient was homozygous for a novel nonsense mutation in exon 4, while the other was a compound heterozygote. Lysyl oxidase (EC 1.4.3.13) is an extracellular copper enzyme with deficient activity in Menkes disease. The levels of lysyl oxidase activity in cultured skin fibroblasts from these Wilson disease patients were also measured.