Genomic DNA was studied from four patients with platelet-type von Willebrand disease (vWD) from two Japanese families previously reported. The entire coding region of platelet glycoprotein (GP) Ib alpha, a component of the platelet receptor for von Willebrand factor (vWF), was examined by polymerase chain reaction (PCR) followed by direct DNA sequence analysis. A single point mutation was found in all patients resulting in substitution of Val (GTG) for Met (ATG) at residue 239 of GPIb alpha. All patients were heterozygous for the mutation, whereas none of the unaffected family members had an amino acid substitution at residue 239. Because the nucleotide substitution destroys an NIa III restriction site on GPIb alpha, PCR products were subjected to digestion with this enzyme; DNA fragments from both normal and mutant alleles were detected in all affected individuals. In allele- specific PCR, DNA was amplified from patients' genomic DNA using either adenine- or guanine-containing primers, whereas only adenine-containing primer successfully amplified DNA from normal individuals. Cloning of amplified DNA into bacteriophage M13mp19 and subsequent DNA sequence analysis confirmed the mutation in these families. The absence of the amino acid substitution at residue 239 of GPIb alpha in the normal individuals tested, together with the linkage of this substitution to the phenotypic expression of disease in these two families and in a family recently described suggest that this amino acid change is a molecular basis for platelet-type vWD, and the substitution may produce a quite similar phenotype to the one reported previously (Gly to Val at residue 233 of GPIb alpha).