We investigated an 8-year-old Arab girl with severe factor XI deficiency; one sibling and her father also have severe factor XI deficiency. Her parents and her father's parents are first cousins. Restriction analysis and DNA sequencing excluded the type I, II, III and IV mutations. We demonstrated a previously undescribed C-->A mutation at nucleotide 1254 in exon 11 resulting in a threonine to asparagine (T-->N) substitution at amino acid 386. We postulate that this substitution interferes with folding and secretion of the molecule.