von Willebrand disease (vWD), the most common inherited bleeding disorder in humans, results from abnormalities in the plasma clotting protein von Willebrand factor (vWF). Severe (type III) vWD is autosomal recessive in inheritance and is associated with extremely low or undetectable vWF levels. We report a method designed to distinguish mRNA expression from the two vWF alleles by PCR analysis of peripheral blood platelet RNA using DNA sequence polymorphisms located within exons of the vWF gene. This approach was applied to a severe-vWD pedigree in which three of eight siblings are affected and the parents and additional siblings are clinically normal. Each parent was shown to carry a vWF allele that is silent at the mRNA level. Family members inheriting both abnormal alleles are affected with severe vWD, whereas individuals with only one abnormal allele are asymptomatic. The maternal and paternal silent alleles are identical at two coding sequence polymorphisms as well as an intron 40 variable number tandem repeat, suggesting a possible common origin. Given the frequencies of the two exon polymorphisms reported here, this analysis should be applicable to approximately 70% of type I and type III vWD patients. This comparative DNA and RNA PCR-restriction fragment length polymorphism approach may also prove useful in identifying defects at the level of gene expression associated with other genetic disorders.