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The mechnism of large platelet formation in Bernard- Soulier Syndrome has not yet been clarified. In this paper we present new morphological findings on large platelets in this disease.Case: A 30 year-old-male. He was admitted to our Department to evaluate hemorrhagic diathesis, because of his repeated nasal bleeding. Methods: l) Platelet retention was measured by the modified Salzman’s method and platelet aggregation test by various agents were performed using a Lumi -Aggregometer. 2) Glycoproteins in the platelet membrane were determined by polyacrylamide gel electrophoresis. 3) Light and electronmicroscopic examinations of the platelet and megakaryocyte were done.Results: Hematological examinations showed a prolonged bleeding time, mild thrombocytopenia, giant platelets in the peripheral blood and normal activities of all plasma clotting factors. The platelet function tests indicated normal platelet aggregation and release of ATP induced by ADP, collagen and thrombin by means of a Lumi-Aggregometer. However, they have markedly decreased by ristocetin and bovine fibrinogen. From these findings, the diagnosis of Bernard-Soulier Syndrome was made. Polyacrylamide gel electrophoresis of platelet membrane in this case showed decreased amount of fraction I in the glycoproteins, that is, a deficiency of negatively charged sialic acid. No abnormalities of megakaryocytes in the bone marrow were found by light microscopy, but electronmicroscopic examinations revealed that some platelets were clearly demarcated into two or three parts by circumferential microtubules.Conclusion: These evidences strongly suggest that giant platelets of Bernard-Soulier Syndrome are made from fusion of membrane of two or three platelets while they circulate in the peripheral blood stream.