Intron 40 of the human von Willebrand factor gene contains a region with variable-number tandem repeats (VNTR), type (ATCT)n, showing length polymorphism. In order to carry out family studies of von Willebrand’s disease, we performed PCR procedures to analyze 3 previously described microsatellites from that region, both in normal individuals and in von WiUebrand disease patients. Three pairs of primers were used to amplify independently nucleotides 1890-1991 (VNTR 1), 2215-2380 (VNTR 2) and 1640-1794 (VNTR 3) from intron 40. The observed heterozygosities (0.75, 0.73 and 0.86 for VNTRs 1, 2 and 3, respectively) were in good agreement with the expected heterozygosities derived from the allele frequencies (0.70,0.73 and 0.79, respectively). Furthermore, the combination of the 3 VNTRs showed 96% of heterozygosity, which correspond with the 98% expected value under linkage equilibrium. Therefore, our conclusion is that the use of these 3 markers, especially VNTR 3, constitutes a rapid and reliable method for performing segregation studies in von WiUebrand disease familie