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Background: The reported prevalence of unruptured cerebral aneurysms (UCAs) varies widely. Objective: To measure the prevalence of UCAs by using 3-dimensional time-of-flight magnetic resonance angiography in adults aged 35 to 75 years. Design: Cross-sectional study done between June 2007 and June 2011. Setting: Two communities chosen at random from 2 districts (1 urban and 1 suburban) in Shanghai, China. Participants: 4813 adults aged 35 to 75 years. Measurements: Three-dimensional time-of-flight magnetic resonance angiography, interpreted by 3 observers blinded to the participants' information, was used to identify the location and size of UCAs and to estimate the overall, age-specific, and sex-specific prevalence. Results: 369 UCAs were found in 336 participants (130 men and 206 women); 4477 participants had no evidence of UCAs. The prevalence was 7.0% overall (95% CI, 6.3% to 7.7%), with 5.5% for men (CI, 4.6% to 6.4%) and 8.4% for women (CI, 7.3% to 9.5%). The overall prevalence of UCAs was higher in women than in men (P < 0.001) and peaked at ages 55 to 64 years in men and women. The UCAs were mostly located in the internal carotid artery (81%), and 90.2% had a maximum diameter less than 5 mm. Mean diameter was larger in women than in men (3.7 mm vs. 3.2 mm; P < 0.009). Limitation: Participants were from 2 communities selected from 2 districts in Shanghai, and adults older than 75 years were not studied. Conclusion: The overall prevalence of UCAs was 7.0% in Chinese adults aged 35 to 75 years, and most lesions had a diameter less than 5 mm. Primary Funding Source: National Natural Science Foundation of China. [ABSTRACT FROM AUTHOR]