The authors report an unusual case of an extensive spontaneous subgaleal hematoma caused by a bilateral arteriovenous shunting lesion involving the diploic veins. The enlarged diploic canals and the existence of bilateral diploic arteriovenous shunts with no history of fracture or trauma indicate that these fistulas developed spontaneously over a long period of time. Angiographic findings in these unusual arteriovenous shunting lesions and their endovascular management are briefly illustrated. A case of a bilateral diploic fistula has not been reported before and it may contribute to the understanding of the diversity of dural arteriovenous shunting lesions.