Sixteen patients with Moyamoya disease and four with quasi-Moyamoya disease were investigated in order to elucidate the presence of thrombophilia. The assay system for diagnosing thrombophilia consisted of assessing both the activity and antigen levels of antithrombin III, protein C, protein S, fibrinogen and plasminogen as well as detecting lupus anticoagulants. The analysis revealed that one third (four definite cases and three quasi-cases) of the examined patients demonstrated either congenital or acquired thrombotic tendency. Protein C deficiency was found in two definite cases and in two quasi-cases among whom one quasi-case was identified to have a hereditary type I Protein C deficiency. Protein S deficiency was found in one definite case and in one quasi-case. Type II plasminogen deficiency was found in one quasi-case, and lupus anticoagulant was present in one quasi-case. Based on these findings, an evaluation of thrombophilia should thus be performed when both diagnosing and treating suspected cases of Moyamoya disease.