Cerebral aneurysm rupture has a poor prognosis, and growing aneurysms are prone to rupture. We therefore conducted a prospective observational study to clarify hemodynamics inducing aneurysm growth, which are poorly understood. Computational fluid dynamics analysis was performed using the patient-specific arterial geometry and flow velocities. Hemodynamic metrics were compared by multivariate analysis between aneurysms enlarged ≥1 mm and stable aneurysms. We enrolled 481 patients. For aneurysms <
4 mm, the time-averaged wall shear stress (WSS) was significantly higher in growing aneurysms for the whole aneurysm, neck, body and parent artery, and transverse WSS was significantly higher on the neck and parent artery. In aneurysms ≥4 mm, the normalized transverse WSS was significantly higher in growing aneurysms for the whole aneurysm and dome. Aneurysms <
4 mm were likely to show whole-aneurysm growth, while aneurysms ≥4 mm were enlarged at the dome. There may exist two hemodynamic mechanisms for aneurysm growth depending on size. Aneurysms <
4 mm may grow near the neck with high magnitudes and multi-directional WSS disturbances, while aneurysms ≥4 mm may grow on the dome with enhanced multi-directional WSS disturbance. These results may be useful in considering indications for surgical treatment. They may help resolve two conflicting hemodynamic rupture theories.