The Bernard-Soulier syndrome (BSS) is a rare congenital bleeding disorder, inherited in an autosomal recessive manner. It is caused by an impaired synthesis or dysfunction of platelet glycoprotein Ib/IX/V complex, which functions as receptor for the von Willebrand factor and thrombin. This paper describes a Polish family with BSS. In three siblings, two symptomatic and one asymptomatic, thrombocytopenia with giant platelets, impaired aggregation to ristocetin and impaired Ib/IX/V complex was diagnosed. Additionally, in each of these three persons, karyotype analysis performed on peripheral blood lymphocytes showed a mosaic presence of supernumerary small marker chromosome (sSMC).