Essential thrombocythemia with acquired von Willebrand's disease.

Two patients with essential thrombocythemia showed a marked decrease in ristocetin-induced platelet aggregation in addition to other aggregation defects during routine aggregation studies for prolonged bleeding times. Further investigation revealed type I von Willebrand's disease (vWD) in one patient and a variant of vWD compatible with type IIA defect in the second patient. The latter patient had minor episodes of epistaxis clinically, while the patient with type I disease experienced excessive blood loss during menstrual periods. Both patients gave negative history for abnormal bleeding in the past or in their families. Both have had uneventful surgical procedures in the past, at which time bleeding times and basic coagulation test results were normal.