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Cook County Illinois ☎ +1 312-443-5500 Last Updated: 08/23/2026 |
| Cook County is the most populous county in the U.S. state of Illinois and the second most populous county in the United States, after Los Angeles County, California. Over 40% of Illinois' total population lives in Cook County. As of 2020, the population was 5,275,541. The county seat is Chicago, the most populous city in Illinois and the third most populous city in the United States. | |
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