Dural arteriovenous fistulas of the transverse/sigmoid sinus usually cause pulse-synchronous bruit but may present catastrophically. Current systems for classifying these vascular malformations do not consider obstruction of venous outflow, which increases the risk of intracranial hemorrhage due to retrograde flow via cortical veins. The authors have developed a grading system based on the severity of venous restrictive disease determined by superselective angiography. In a retrospective analysis of 25 patients with dural arteriovenous fistulas of the transverse/sigmoid sinus treated between 1988 and 1990, the grade of venous restrictive disease reflected the clinical presentation. Visual symptoms and central nervous system hemorrhage were more common in patients with cortical venous drainage and more severe distal venous occlusion (Grade 3: 31% and 31%, respectively; Grade 4: 67% and 100%, respectively) than in patients with cortical venous drainage and mild-to-moderate venous restrictive disease (Grade 2: 13% and 0%, respectively) or those without venous outflow (Grade 1: 0% and 0%, respectively). These preliminary results suggest that this grading system may be useful for predicting the risk of catastrophic clinical presentation and for guiding therapeutic decision-making in patients with dural arteriovenous fistulas of the transverse/sigmoid sinus. A prospective study of a larger number of patients is needed to validate the predictive value of this new grading system.