To determine when and how intracranial aneurysms causing mass effect change following endovascular treatment, we used MRI to assess patients for 2-3 years after the interventional procedure. Nine patients who had aneurysms compressing the surrounding structures underwent endovascular treatment. Proximal occlusion of the parent artery was performed in seven cases, and in two the aneurysm was embolised with microcoils. After embolisation, signal intensity within aneurysms tended to be high on both T1- and T2-weighted images. When there was rapid reduction in size high-signal zones within aneurysms became isointense or gave low signal on T1-weighted images. On T2-weighted images, isointense or low-signal foci appeared within high-signal areas in the aneurysm, giving mixed intensity. In typical cases, the mean volume of the aneurysm fell to approximately 30% of its initial value 2-12 months after treatment. After this, no additional reduction was observed. The aneurysms which showed little signal intensity change tended to shrink more slowly and to a lesser degree than the more typical cases. Aneurysms which gave high signal on both T1- and T2-weighted images early following embolisation shrank more quickly than those showing little signal change.