Purpose To investigate whether sinovenous outflow restriction (SOR) is more strongly associated with hemorrhage than cortical venous reflux (CVR) in patients with lateral sinus dural arteriovenous fistulas (DAVFs). Materials and Methods An institutional review board approved this retrospective study and waiver of informed consent was obtained. From 1995 to 2016, 163 cases of lateral sinus DAVFs were included and divided into hemorrhagic and nonhemorrhagic groups based on initial presentation. Their angiograms and magnetic resonance images were evaluated, with two evaluators independently grading CVR and SOR. The SOR was scored as the combined conduit score (CCS), ranging from zero (total occlusion) to 8 (fully patent). The CVR and CCS of the hemorrhagic and nonhemorrhagic groups were compared. Logistic regression models were established for both the CVR and CCS to compare their performances in discriminating DAVF hemorrhage. Results Sinovenous outflow was significantly more restrictive (lower median CCS) in the hemorrhagic group than in the nonhemorrhagic group (1 vs 6.5; P < .001). A CCS of less than or equal to 2 best discriminated between the groups with a sensitivity of 90.0% and a specificity of 88.1%. The CCS model had a higher discriminative performance than did the CVR model (area under the curve, 0.933 vs 0.843; P = .018). Conclusion The CCS grading system semiquantifies SOR. SOR may represent a stronger risk factor associated with hemorrhage in patients with lateral sinus DAVFs than does CVR, and thus may offer guidance in therapeutic decision making. © RSNA, 2017.