A 25-year-old Japanese man was diagnosed as having primary pulmonary hypertension, on the basis of the findings of cardiac catheterization and ventilation-perfusion scintiscans. The plasma level of plasminogen in this patient was found to be reduced to 43% by a functional assay and 61 mg/l by an antigenic assay. Based on the family study, the patient was considered to have a heterozygous congenital plasminogen deficiency. Accordingly, it is suggested that the defective fibrinolysis in this patient may have played an important role in the development of primary pulmonary hypertension through microthrombosis.