The simultaneous occurrence in the same patient of an intracranial saccular aneurysm and an arteriovenous malformation (AVM) is a well-known phenomenon. Usually the aneurysms are related anatomically to the arteries supplying the AVM, and it is generally accepted that the aneurysms are caused by hemodynamic stresses resulting from the presence of an AVM. Because patients with both an AVM and an aneurysm are older than those presenting with an AVM alone, a time factor seems essential in the development of the aneurysm accompanying an AVM. In this article, the case reports of two children are presented. They both had a symptom-producing AVM and an attendant saccular aneurysm. The malformations were anatomically closely related and the significance of hemodynamic stresses in the development of the aneurysms cannot be neglected. However, in these two cases, the time factor obviously cannot be of vital importance. Therefore, another factor, possibly in the form of a vascular collagen defect, may be suspected as essential in the formation of aneurysms during childhood. The character of this defect is briefly discussed.