Background: The M2-360° is a recent class of aneurysm coil. This device combines the second generation of bioactive copolymer coating, which is intended to promote aneurysm fibrosis, with the "360°" design, which is meant to improve uniformity and density of packing. This study evaluates the safety and angiographic stability of these devices. Methods: This was a retrospective review of 86 consecutive patients with 100 intracranial aneurysms that were treated using M2-360°s. Follow-up was done at 6 and 12 months. Results: Seventy-eight aneurysms were coiled solely with M2-360°s, and 22 aneurysms were treated with a combination of coils. In mixed-coil cases, the average percentage of coil volume consisting of M2-360° coils was 78%. Procedure-related neurologic complications occurred in 6 patients (7%). Initial complete occlusion was obtained in 80 aneurysms. Of 76 aneurysms with 6-month angiographic follow-up, 4 (5.3%) revealed further occlusion, 54 (71.1%) were unchanged, and 18 (23.7%) showed recanalization. Of 38 aneurysms with 12-month follow-up, 1 (2.6%) revealed further occlusion, 23 (60.5%) were unchanged, and 14 (36.8%) showed recanalization. Six- and 12-month angiograms showed major recanalization (requiring further coiling) in 3.9% and 15.8% of cases, respectively. Conclusions: The risk of complications with M2-360°-treated aneurysms is comparable with reports of other coils, indicating that M2-360°s are relatively safe. Although the initial occlusion rate is higher than that in other coiling series, recanalization rates were similar to those obtained with other coil designs. This study does not demonstrate an advantage with M2-360°s. © 2009 Elsevier Inc. All rights reserved.