We have previously demonstrated within intron 40 of the von Willebrand factor (vWF) gene a region of ATCT repeats that was shown to vary in length between two different DNA clones from unrelated individuals. The polymerase chain reaction (PCR) was used to examine the variability in length of this variable number tandem repeat (VNTR) in 53 normal individuals, using primers to DNA sequence flanking the repeat region. Overall, eight different length allelic bands were seen. These were individually sequenced and shown to contain from 6 to 14 ATCT repeats (a nine-repeat band was not seen). Seventy-five percent of individuals were shown to be heterozygous for this vWF.VNTR, and family studies showed Mendelian inheritance with allelic frequencies from 1% (vWF.VNTR [8] and vWF.VNTR [14]) to 39% (vWF.VNTR [7]). In the family of a patient with type III severe von Willebrand disease (vWD), vWF.VNTR results mirrored the phenotypic data and results with previously reported intragenic vWF restriction fragment length polymorphisms (RFLP). The patient was shown to be a compound heterozygote. In a family with a child with severe type III vWD, prenatal diagnosis by vWF.VNTR analysis on DNA obtained by chorionic villus sampling at 10 weeks gestation during a subsequent pregnancy indicated a severely affected fetus. This diagnosis was confirmed by fetal blood sampling at 18 weeks.