Recently, therapeutic embolization has been advocated as the treatment of choice for arteriovenous malformations (AVM's) of the spine. However, no study has established lasting benefit from this procedure or determined the incidence of recanalization, as occurs with cerebral AVM's. In this study, six patients were followed periodically after complete obliteration of their AVM's by particulate embolization was shown by immediate arteriography. The study group included three men (aged 59 to 72 years) with spinal dural arteriovenous (AV) fistulas and three women (aged 27 to 38 years) with intramedullary glomus-type spinal cord AVM's. The patients were treated by embolization with 100- to 1000-microns diameter polyvinyl alcohol particles. Clinical improvement, most commonly manifesting as increased lower-extremity strength, occurred in all patients after embolization. However, recurrent symptoms, including weakness, numbness, and urinary incontinence, occurred within 2 and 8 months in two of the three patients with dural AV fistulas and within 2 months in two of the three patients with glomus AVM's, prompting radiological reevaluation. Spinal arteriography revealed recanalization of the AV fistulas and spinal AVM's in five patients. Magnetic resonance (MR) imaging demonstrated a signal-void area caused by intramedullary AVM's. This area disappeared after embolic occlusion, but recurred after delayed recanalization, indicating restored flow through the AVM. Embolization provides only temporary treatment for many spinal AVM's. After embolic occlusion, delayed reassessment with arteriography and/or MR imaging is indicated, particularly if the symptoms persist or recur. Surgical excision of spinal AVM's provides the only therapeutic means to eliminate flow through the AVM permanently in most patients, and should be considered the treatment of choice when feasible.