This study investigated the natural history and biological behavior of incidental fusiform aneurysms in four patients with incidental fusiform aneurysms of the vertebrobasilar arterial system who had been followed up for more than 3 years (mean 3.5 years). Two lesions remained the same size, and two lesions gradually grew. Angiography showed the non-growing fusiform aneurysms as a circumferentially or unilaterally fusiform dilatation of a short segment of the vertebral artery with smooth walls and a steep slope of the dilatation, and the growing fusiform aneurysms as unilaterally fusiform involving a long segment of the vertebral artery or basilar artery with irregular walls and a gentle slope of dilatation. Magnetic resonance (MR) imaging demonstrated the non-growing fusiform aneurysms as a signal-void area, and the growing fusiform aneurysms as high and intermediate signals in addition to the normal flow void. The heterogeneous MR intensities probably correspond to turbulent flow, laminar flow, thrombosis, or intramural hematoma. Differentiation of growing and non-growing fusiform aneurysms is very difficult at the initial diagnosis. However, enlargement of the fusiform aneurysms is consistent with hemorrhage into the aneurysmal wall, which is confirmed by MR imaging. Fusiform aneurysms with the characteristics of the growing aneurysms cannot be overlooked because of the potential to develop into giant fusiform aneurysms which are very difficult to manage therapeutically.