Summary. Sixty patients with von Willebrand's disease belonging to 34 unrelated families were classified into the various types of the disease by analysis of the multimer patterns of von Willebrand factor. Type I disease was observed in 62% of the families, a finding similar to the recently published British and Swedish series of patients. Type III (severe) von Willebrand's disease was observed more frequently in the Israeli group (29%) than in the other two series. Of the 16 patients with type III disease eight were Arabs living in Israel, the West Bank and the Gaza Strip (total population about 1·5 × 106). Thus, it appears that the frequency of type III disease is very high among Arabs (5·3/106). Sibship analysis of all families affected by the type III mutant gene was compatible with an autosomal recessive mode of in heritance. An attempt was made to define carriers of type III disease and to distinguish them from type I patients and from healthy subjects. In 15 obligate carriers of type III disease mean levels of factor VIII clotting activity, of von Willebrand factor and of ristocetin cofactor were significantly higher than the corresponding mean values observed in 31 symptomatic and 12 asymptomatic type 1 patients, and in turn significantly lower than the corresponding mean values observed in 30 healthy subjects. Ristocetin cofactor was the best criterion for discrimination among carriers of type III disease, normal subjects and type I patients.