A 6-year-old male with vertebral-basilar artery thrombosis was recognized to have high-molecular-weight kininogen (HK) deficiency. The propositus had no HK procoagulant activity and antigen (< 1%). Using monoclonal antibodies (Mabs) to kininogen domain 3, the propositus, family members, and Fitzgerald plasma were determined to have detectable low-molecular-weight kininogen. Mabs to HK domains 5 and 6 do not detect HK antigen in the propositus' plasma. The propositus has a single base pair (bp) deletion in cDNA position 1492 of exon 10 affecting amino acid 480 of the mature protein and resulting in a frameshift and a premature stop codon at position 1597 (amino acid 532). Unexpectedly, Mabs to the heavy chain and domain 5 of HK detect a 92-kDa form of HK in Fitzgerald plasma, the first HK-deficient plasma. The 92-kDa Fitzgerald HK has amino acid residues through 502, corresponding to domains 1 through 5, but lacks epitopes of domain 6 (positions 543 to 595). Fitzgerald DNA has a normal exon 10, but a 17-bp mutation in intron 9. These combined results indicate that mutations in the kininogen gene may differentially affect biosynthesis, processing, and/or secretion of HK.