We report a case of congenital and familial antithrombin III deficiency developing massive pulmonary thromboembolism. A 44-year-old man was admitted to our hospital because of sudden chest pain and severe dyspnea. Five years ago, he was operated due to a mesenteric vein thrombosis of unknown cause. On admission, radioisotopic venogram showed deep vein thrombosis and lung scintigram showed multiple segmental perfusion defects. His plasma antithrombin III level was 10.5 mg/dL which was less than 50% of normal and those of a son and two daughters were also decreased. After treatment with tissue plasminogen activator, heparin and coumadin, his symptom and lung scintigram were significantly improved. As far as we reviewed, there were very rare reports with congenital antithrombin III deficiency presenting as pulmonary thromboembolism in Korea.