Unruptured intracranial aneurysms (UIAs) are increasingly detected and are an important health-care burden. We aimed to assess the prevalence of UIAs according to family history, comorbidity, sex, age, country, and time period. Through searches of PubMed, Embase, and Web of Science we updated our 1998 systematic review up to March, 2011. We calculated prevalences and prevalence ratios (PRs) with random-effects binomial meta-analysis. We assessed time trends with year of study as a continuous variable. We included 68 studies, which reported on 83 study populations and 1450 UIAs in 94 912 patients from 21 countries. The overall prevalence was estimated as 3·2% (95% CI 1·9-5·2) in a population without comorbidity, with a mean age of 50 years, and consisting of 50% men. Compared with populations without the comorbidity, PRs were 6·9 (95% CI 3·5-14) for autosomal dominant polycystic kidney disease (ADPKD), 3·4 (1·9-5·9) for a positive family history of intracranial aneurysm of subarachnoid haemorrhage, 3·6 (0·4-30) for brain tumour, 2·0 (0·9-4·6) for pituitary adenoma, and 1·7 (0·9-3·0) for atherosclerosis. The PR for women compared with men was 1·61 (1·02-2·54), with a ratio of 2·2 (1·3-3·6) in study populations with a mean age of more than 50 years. Compared with patients older than 80 years, we found no differences by age, except for patients younger than 30 years (0·01, 0·00-0·12). Compared with the USA, PRs were similar for other countries, including Japan (0·8, 0·4-1·7) and Finland (1·0, 0·4-2·4). There was no statistically significant time trend. The prevalence of UIAs is higher in patients with ADPKD or a positive family history of intracranial aneurysm of subarachnoid haemorrhage than in people without comorbidity. In Finland and Japan, the higher incidence of subarachnoid haemorrhage is not explained by a higher prevalence of UIAs, implicating higher risks of rupture. Julius Centre for Health Sciences and Primary Care and Department of Neurology and Neurosurgery, University Medical Centre, Utrecht. Copyright © 2011 Elsevier Ltd. All rights reserved.