A syndrome of retroperitoneal hemorrhage during anticoagulant therapy associated with nerve involvement in femoral nerve or lumbar plexus is described. Twenty-one cases were collected from the English-language literature, and five new cases were added for review and analysis. In these 26 cases spontaneous hemorrhage occurred under iliopsoas fascia in the retroperitoneal space. The clinical picture, involved nerves, laboratory findings of coagulation studies, anemia, and the outcome of these cases are summarized and discussed. Retroperitoneal space may contain a large quantity of internal hemorrhage. Irreversible damage of lumbar plexus or femoral nerve may result from entrapment of nerves in the hematoma. Laboratory coagulation studies may guide effective administration of anticoagulant therapy and minimize hemorrhagic complication, but they will not eliminate the risk of hemorrhage completely.