Background and purpose: The use of Onyx has become the mainstream for the treatment of cranial dural arteriovenous fistulas (AVFs) and arteriovenous malformations, but the reported success for type I spinal dural arteriovenous fistulas (sDAVFs) remains limited. We review our experience with Onyx and report its limitations in the treatment of spinal AVFs. Materials and methods: We retrospectively reviewed the Interventional Neuroradiology Procedure database at Washington University for cases of sDAVF embolization. Radiology reports were reviewed for fistula classification, treatment technique, and initial and follow-up results. Angiographic images were reviewed to confirm diagnosis, treatment, and penetration of embolisate into the draining vein. Results: With the use of Onyx, sDAVFs were obliterated in six of seven patients at the time of treatment. Follow-up angiography confirmed sDAVF obliteration in two patients, and recurrence in two cases. Two patients had no follow-up. One patient not cured at the time of treatment was treated surgically. Of the nine total treatments, Onyx successfully crossed the nidus into the draining vein in only four cases. Successful venous embolization was facilitated with positioning of the microcatheter to less than 5 mm from the nidus in three of the four cases. The use of n-butyl cyanoacrylate (NBCA) resulted in venous penetration in eight of 10 cases, and short term follow-up cure in seven of 10 patients. Conclusions: Our experience with Onyx for type I sDAVF embolization has been tempered by difficulty in achieving venous penetration and, consequently, a high rate of recurrence. For management of these fistulas, we favor NBCA or surgical treatment.