An Arg91Gln substitution in the mature von Willebrand factor (vWF) has been associated with defective binding of vWF to factor VIII (FVIII). We studied four families with members initially classified as having type I von Willebrand disease (vWD) who were either homozygous or heterozygous for the Arg91Gln change. The first family was the original case described by Nishino et al. (1) where three members were homozygous for the Gln91 allele. They had a low FVIII coagulant activity:vWF antigen (VIIIC:vWFAg) ratio, from 0.29 to 0.44, and the ability of their plasma vWF to bind FVIII was markedly decreased. All the heterozygous members had normal vWF and FVIII levels but the capacity of their plasma vWF to bind FVIII was reduced and intermediate between the homozygous members and normals. The affected individual from the second family was heterozygous for the Gln91 allele and demonstrated a VIIIC:vWFAg ratio of 0.98. The FVIII binding assay confirmed the heterozygous status indicating that the moderately low levels of vWF were due to reduced expression of both alleles. The propositus from the third family was also heterozygous and had below normal levels of vWF as well as a low VIIIC:vWFAg ratio of 0.34; however, FVIII binding to her plasma vWF was similar to that of the homozygous individuals suggesting that Gln91-vWF was the major circulating form. Her daughter who has type I vWD inherited the allele without the Gln91 mutation indicating that the expression of this allele was indeed impaired. The heterozygous patient in the fourth family had a vWF level of 24 U/dl but an VIIIC:vWFAg ratio greater than 2.(ABSTRACT TRUNCATED AT 250 WORDS)