We identified a mutation in the ceruloplasmin (Cp) gene in a Japanese family with aceruloplasminemia, some of whose members showed extrapyramidal disorders, cerebellar ataxia, and diabetes mellitus. A post-mortem study of the proband revealed excessive iron deposition mainly in the brain, liver and pancreas. The G to A transition at the splice acceptor site introduces a premature termination codon at the amino acid position 991 by defective splicing, thereby truncating the carboxyl terminus of Cp in affected individuals. We conclude that the mutation in the Cp gene is associated with systemic hemosiderosis in humans.