question: Transvenous embolization is an effective method for treating dural arteriovenous fistulas (DAVFs) of the transverse-sigmoid sinus (TSS). However, in cases of complicated DAVFs, it is difficult to preserve the patency of the dural sinus. The authors describe the technical details of a new reconstructive technique using transvenous balloon-assisted Onyx embolization as another treatment option in a patient with an extensive and complex DAVF of the left TSS. A microcatheter and compliant balloon catheter were navigated into the left internal jugular vein and placed at the distal end of the DAVF in the transverse sinus. The microcatheter was placed between the vessel wall of the TSS and the balloon. After the balloon was fully inflated, Onyx-18 was injected at the periphery of the balloon in a slow, controlled, progressive, stepwise manner; the balloon and microcatheter were simultaneously withdrawn toward the sigmoid sinus, with Onyx encompassing the entirety of the complex DAVF. The Onyx refluxed into multiple arterial feeders in a distal-to-proximal step-by-step manner, ultimately resulting in an Onyx tunnel. The final angiography study revealed complete obliteration of the DAVF and patency of the TSS. The Onyx tunnel, or reconstructive transvenous balloon-assisted Onyx embolization technique, may be an effective treatment option for large, complex DAVFs of the TSS. This technique may provide another option to facilitate the complete obliteration of the DAVF while preserving the functional sinus.