A case of Bernard-Soulier syndrome in a five-year-old female is presented. The diagnosis was confirmed by flow cytometric analysis of glycoprotein Ib (CD 42b) in addition to the patient's classic laboratory findings such as prolonged bleeding time, mild thrombocytopenia, large platelets and failure of platelet aggregation with ristocetin. Her parents and sibling had normal coagulation tests and CD 42b levels. It is emphasized that flow cytometric analysis is useful in the confirmation of congenital platelet function defects.