Patients with a low platelet count are prone to bleeding. The occurrence of a thrombotic event in congenital thrombocytopenic patients is rare and puzzling. At least nine patients with Glanzmann thrombasthenia have been reported to have had a thrombotic event, eight venous and one arterial (intracardiac, in the left ventricle). On the contrary, three patients with Bernard-Soulier syndrome have been shown to have had arterial thrombosis (myocardial infarction) but no venous thrombosis. Finally, seven patients with the familiar macrothrombocytopenia due to alterations of the MYH9 gene have been reported to have had thrombosis (five myocardial infractions, one ischemic stroke, one deep vein thrombosis and one portal vein thrombosis). The significance of these findings is discussed with particular emphasis on the discrepancy between venous and arterial thrombosis seen in patients with Glanzmann thrombasthenia and Bernard-Soulier syndrome.