The proportion of incompletely occluded aneurysms after coiling varies widely between studies. To assess overall outcome of coiling, we systematically reviewed the literature to determine initial occlusion, reopening, and retreatment rates of coiled aneurysms according to predefined criteria and subgroups. We searched PubMed and EMBASE (January 1999 to September 2008) for studies of >50 coiled aneurysms. Two reviewers independently extracted data. We grouped studies reporting on only ruptured aneurysms, posterior circulation aneurysms, and studies with large proportions of aneurysms >10 mm to assess possible determinants for incomplete occlusion, reopening, and retreatment. Forty-six studies totalling 8161 coiled aneurysms met inclusion criteria. Immediately after coiling, 91.2% (95% CI, 90.6% to 91.9%) of the aneurysms were adequately occluded. Aneurysm reopening occurred in 20.8% (95% CI, 19.8% to 21.9%) and retreatment was performed in 10.3% (95% CI, 9.5% to 11.0%). Reopening rate was lower in studies reporting on ruptured aneurysms only compared with all studies (11.4% versus 20.8%; relative risk, 0.55; 95% CI, 0.47 to 0.64) and higher in studies focusing on posterior circulation aneurysms compared with studies with >85% anterior circulation aneurysms (22.5% versus 15.5%; relative risk, 1.5; 95% CI,1.2 to 1.7). Regression analysis showed higher retreatment rates with increasing proportion of aneurysms >10 mm (beta=0.252; 95% CI, 0.073 to 0.432). We could not find a relation between reopening and type of coils used. At follow-up, one fifth of all coiled intracranial aneurysms shows reopening of which half is retreated. Possible risk factors for aneurysm reopening are location in the posterior circulation and size >10 mm. To confirm our findings, a meta-analysis on individual well-reported patient data is desirable.