Spontaneous intra-aneurysmal thrombosis occurs in approximately 50% of giant intracranial aneurysms. The incidence of this process is associated to location, size, and origin, and the natural history of spontaneous thrombosis occurrence in non-giant aneurysms is rare and still unclear. We describe two non-giant middle cerebral artery (MCA) aneurysms that spontaneously thrombosed and comment the aspects of the literature.