Wilson disease (WD) is an autosomal recessive disorder characterized by copper accumulation in the liver, brain, kidneys, and corneas, and culminating in copper toxication in these organs. In this study, we analyzed mutations of the responsible gene, ATP7B, in four Japanese patients with WD. By direct sequencing, we identified five mutations, of which two were novel, and 16 polymorphisms, of which 6 were novel. The mutations 2871delC and 2513delA shift the reading frame so that truncated abnormal protein is expected. In contrast to these mutations found in patients with hepatic-type of early onset, the mutations A874V, R778L, and 3892delGTC were either missense mutations or in frame 1-amino acid deletion, and occurred in the patients with hepato-neurologic type of late onset. The mutations 2871delC and R778L have been previously reported in a relatively large number of Japanese patients. In particular, R778L is known to be more prevalent in Asian countries than in other countries of the world. Our data are compatible with the hypothesis that the mutations tend to occur in a population-specific manner. Therefore, the accumulation of the types of mutations in Japanese patients with WD will facilitate the fast and effective genetic diagnosis of WD in Japanese patients.