We identified a novel mutation in an asymptomatic 65-year-old Japanese man with severe factor XI deficiency. Sequence analysis after polymerase chain reaction single-stranded conformation polymorphism (PCR-SSCP) analysis of his factor XI gene revealed a G-->A transition in codon 501 of exon 13, resulting in a substitution of Trp501 (TGG) by a stop codon (TAG) in the catalytic domain. This mutation abolished a FokI restriction site. The PCR product from normal subjects was digested with FokI and yielded two fragments, one of 223 bp and one of 47 bp. The PCR product from the patient gave a single 270-bp fragment, demonstrating possible homozygosity.