A 66-year-old man was admitted to our hospital because of progressive dyspnea on effort. Arterial blood gas analysis showed severe hypoxemia, and a chest radiograph revealed reticular shadows in both lower lungs and an increase of the cardiothoracic ratio. Echocardiography demonstrated mild indentation of the interventricular septum toward the left ventricle, moderate pericardial effusion and pulmonary hypertension. From these data, we diagnosed pulmonary thromboembolism and started anticoagulation therapy. After the addition of the administration of warfarin and oxygen therapy, his symptoms disappeared. However, we could not obtain more supporting evidence of thromboembolization by methods of ventilation-perfusion scanning, digital subtraction angiography of the pulmonary artery, or venography. Blood coagulation analysis demonstrated that the patient's plasma protein C antigen levels and its activity were depleted. The patient's son had a history of thrombophlebitis and pulmonary embolization, and his data of protein C antigen levels was also decreased. Therefore, this patient was found to have a character of familial protein C deficiency type I. We could not get the conclusive proof of pulmonary thromboembolism, but we considered that the presence of familial protein C deficiency may cause exacerbation of pulmonary hypertension.