The correlation between volume embolization ratio (VER), the percentage of aneurysm volume occupied by coils, and aneurysm stability after endovascular treatment with platinum coils was studied in 86 patients who underwent embolization of 88 cerebral saccular aneurysms with standard platinum coils between March 1997 and January 2003. Radiographic follow up was possible of 62 aneurysms with greater than 70% obliteration on immediate post-procedure angiograms for more than 6 months. Immediate post-procedure evaluation found that 12 of 14 small (<4 mm), 12 of 38 medium (4-<10 mm), and two of 10 large (> or =10 mm) aneurysms had high (> or =25%) VER. In addition, the VER was high in 22 of 40 aneurysms with small (<4 mm) and four of 22 with wide necks (> or =4 mm). Evaluation of 57 aneurysms at 6 months after coil embolization found recanalization in two of 25 aneurysms with high VER and 21 of 32 with low (<25%) VER, indicating that aneurysms with high VER are significantly more stable than those with low VER (p < 0.01). VER strongly affects aneurysm stability, so high VER is desirable and a useful predictor of aneurysm stability.