Protein C is the central component of the major anti-thrombotic regulatory system, and individuals with hereditary protein C deficiency tend to have an increased risk of thromboembolism. During the last several years, mutations causing protein C deficiency have been identified. In the present study, we report familial cases with three nucleotide substitutions: One is a missense mutation Arg169Trp, which was previously reported. The other two are C-154T promoter polymorphism (rs1799808 on dbSNP database), the function of which is unknown and Ser99Ser synonymous polymorphism (rs5936). All three mutations were found in a 24-year-old patient with pulmonary thromboembolism and his 54-year-old father who also had pulmonary thromboembolism. C-154T promoter polymorphism (rs1799808 on dbSNP database) and Ser99Ser synonymous polymorphism (rs5936) were found in the patient's mother.