A 27-year-old female patient presented with headache, vomiting, and visual disturbances who was evaluated and detected to have a direct carotid cavernous fistula (CCF). Secondary causes were ruled out, and she was treated with coil occlusion and glue injection. A month after almost complete clinical recovery, she developed deep vein thrombosis of left thigh. Subsequent work-up revealed antithrombin III (ATIII) deficiency in her. To the best of our knowledge, this is the first reported case of ATIII deficiency associated with CCF. This case shows the importance of working up for a primary etiology if any, to prevent further complications after surgery.