Flow diversion (FD) is effective for treatment of intracranial internal carotid artery (ICA) aneurysms. To determine whether carotid siphon (CS) geometry influences the efficacy of FD when employed for ICA aneurysms. Outcomes of a consecutive series of patients with ICA aneurysms treated with FD were retrospectively reviewed. CS anatomy was quantified through measurement of the posterior, anterior, and anterosuperior bend angles in accordance with previously described methodology. The relationship of CS geometry to likelihood of incomplete aneurysm occlusion at 1 yr after treatment was assessed with multivariate logistic regression analysis. There were 167 ICA aneurysms in 164 patients treated with FD during the study period. The mean age of our cohort was 55.7 yr (standard deviation [SD]: 12.3) and a majority of patients were female (145/164, 86.8%). Anterior (47.4 degrees vs 8.5, P < .001) and anterosuperior bend angles (100.9 vs 76.5, P = .002) were significantly greater in aneurysms that required repeat FD after initial treatment. On multivariate logistic regression analysis, increasing patient age (unit odds ratio [OR]: 1.05, 95% confidence interval [CI] 1.01-1.08; P = .003) and anterior bend angle ≥-3.5 (OR: 2.47, 95% CI 1.04-5.86; P = .046) were associated with increased odds of incomplete aneurysm occlusion at 1 yr after treatment. These findings suggest that variations in CS anatomy may influence the efficacy of FD treatment and should be analyzed prior to offering FD for treatment of ICA aneurysms. Further investigation into the hemodynamic effects of CS geometry is warranted.