Background and Purpose—Basilar artery (BA) bifurcation aneurysms are common, but the correlation between BA bifurcation morphology and aneurysm formation remains to be established. Our purpose was to determine the association of BA bifurcation aneurysms with patient age, sex, bifurcation angle, and branch diameter.Methods—Three-dimensional angiographic data of 195 patients were used, including 59 patients with BA bifurcation aneurysms and 136 control subjects. The angles formed between left and right posterior cerebral arteries (φ1) and between posterior cerebral artery and BA (the smaller angle defined as φ2 and the larger one as φ3), arterial diameters, and BA bifurcation aneurysm geometric characters were examined.Results—Women of 40 to 70 years of age are more vulnerable to BA bifurcation aneurysm formation than men. The φ1 bifurcation angle significantly increased (P<0.0001), whereas both φ2 and φ3 angles significantly decreased (P<0.0001 and P=0.09, respectively) with increase of patients’ age. Statistically significant (P<0.0001 and P=0.0002, respectively) positive correlations were observed between BA bifurcation branch diameter and aneurysm size. The φ1 angle was significantly (P<0.0001) wider in patients harboring BA bifurcation aneurysms than the control, whereas φ2 and φ3 angles in aneurysm group were significantly smaller than those in the control group (P<0.0001). The BA bifurcation aneurysms were mostly deviated toward the smaller φ2 angle side between φ2 and φ3 angles and deviated toward the smaller-diameter daughter posterior cerebral artery branch.Conclusions—BA bifurcation aneurysms are significantly associated with patients’ age, female sex, wider bifurcation angles, and smaller vascular diameter at the BA bifurcation.