The objectives of this study were to elucidate the normal anatomy of middle cerebral artery (MCA) bifurcations and to analyze the differences in patients with MCA aneurysms. In the present study, 62 patients underwent three-dimensional magnetic resonance angiography, and no intracranial lesions were noted. The widths of M1 and the superior and inferior M2 branches, as well as their respective lateral angles, were measured. These values were used to calculate the daughter artery ratio (DA ratio; width of larger M2/width of smaller M2) and the lateral angle ratio (LA ratio; lateral angle between M1 and larger M2/lateral angle between M1 and smaller M2). The DA and LA ratios of 54 MCA aneurysm patients (34 with ruptured aneurysms, 20 with unruptured aneurysms) were also calculated, using three-dimensional digital subtraction angiography, and compared with the normal values. In normal patients, the widths of M1 and the branches of M2, the lateral angles, and the LA and DA ratios were not significantly different between the right and left sides. The bilateral superior and inferior lateral angles of normal MCAs were significantly wider than those of MCAs with aneurysms. The DA ratio was 1.5 ± 0.4 in normal MCAs and 1.7 ± 0.7 in MCAs with aneurysms; this difference was significant (p < 0.05). The LA ratio was 1.3 ± 0.4 in normal MCAs and 2.1 ± 1.4 in MCAs with aneurysms; these values were also significantly different (p < 0.01). Normal cerebral artery bifurcations show close to symmetric structure in the M2 branches and the lateral angles, whereas aneurysmal MCAs do not show this symmetry.