Onyx migration in the endovascular treatment of dural arteriovenous fistulas (dural AVFs) is uncommon. We describe five cases of Onyx migration to the heart and draining vein and its avoidance. Between February 2007 and August 2008, Onyx migration was encountered in five patients with dural AVFs treated endovascularly at our institute. Procedures performed under general anesthesia consisted of two arterial approaches and three venous approaches. Two patients with dural AVFs involving the transverse-sigmoid sinus were treated by transarterial embolization using Onyx-18 via the occipital artery and the posterior branch of the middle meningeal artery, respectively. A piece of Onyx was found in the right ventricle on post-embolization chest X-ray film in both patients, one developed tricuspid valve dysfunction requiring thoracic surgery and one was asymptomatic. The other three patients were treated with a combination of Onyx (34 or 18) and coils transvenously with venous Onyx migration leading to draining vein occlusion, one with dural AVF involving the tentorium died from venous rupture, two patients with bilateral dural AVFs of the cavernous sinus (one with deterioration of ocular symptoms and one without symptoms). Postoperative digital subtraction angiography confirmed the elimination of dural AVF in one patient, and residual fistulae in three patients. The follow-up study ranging from two to nine months (average, 4.5 months). Three patients recovered to their full activities, while one had visual disturbance. Although Onyx has been considered a controllable embolic agent, its migration to other locations causing clinical deterioration can occur. This problem should be noted and prevented.