We investigated an asymptomatic 19-year-old patient with factor XI deficiency diagnosed in the context of presurgical laboratory screening. The F11 gene was analyzed and a novel missense mutation I463S in exon 12 was identified in heterozygosity in the proband. His mother, also diagnosed with asymptomatic factor XI deficiency, was found to be heterozygous for the same mutation. This novel amino acid substitution in the serine protease catalytic domain appears to be responsible for the low factor XI levels in both individuals.