U.S. Population – County Population
| Rank | County | 2008 Estimate | 2000 Census | Growth (%) |
| 1 | Los Angeles County, CA | 9,862,049 | 9,519,338 | 3.6 |
| 2 | Cook County, IL | 5,294,664 | 5,376,741 | -1.5 |
| 3 | Harris County, TX | 3,984,349 | 3,400,578 | 17.2 |
| 4 | Maricopa County, AZ | 3,954,598 | 3,072,149 | 28.7 |
| 5 | Orange County, CA | 3,010,759 | 2,846,289 | 5.8 |
| 6 | San Diego County, CA | 3,001,072 | 2,813,833 | 6.7 |
| 7 | Kings County, NY | 2,556,598 | 2,465,326 | 3.7 |
| 8 | Dallas County, TX | 2,412,827 | 2,218,899 | 8.7 |
| 9 | Miami-Dade County, FL | 2,398,245 | 2,253,362 | 6.4 |
| 10 | Queens County, NY | 2,293,007 | 2,229,379 | 2.9 |
| 11 | Riverside County, CA | 2,100,516 | 1,545,387 | 35.9 |
| 12 | San Bernardino County, CA | 2,015,355 | 1,709,434 | 17.9 |
| 13 | Wayne County, MI | 1,949,929 | 2,061,162 | -5.4 |
| 14 | King County, WA | 1,875,519 | 1,737,034 | 8.0 |
| 15 | Clark County, NV | 1,865,746 | 1,375,765 | 35.6 |
| 16 | Santa Clara County, CA | 1,764,499 | 1,682,585 | 4.9 |
| 17 | Broward County, FL | 1,751,234 | 1,623,018 | 7.9 |
| 18 | Tarrant County, TX | 1,750,091 | 1,446,219 | 21.0 |
| 19 | New York County, NY | 1,634,795 | 1,537,195 | 6.3 |
| 20 | Bexar County, TX | 1,622,899 | 1,392,931 | 16.5 |
| 21 | Suffolk County, NY | 1,512,224 | 1,419,369 | 6.5 |
| 22 | Middlesex County, MA | 1,482,478 | 1,465,396 | 1.2 |
| 23 | Alameda County, CA | 1,474,368 | 1,443,741 | 2.1 |
| 24 | Philadelphia County, PA | 1,447,395 | 1,517,550 | -4.6 |
| 25 | Sacramento County, CA | 1,394,154 | 1,223,499 | 13.9 |
| 26 | Bronx County, NY | 1,391,903 | 1,332,650 | 4.4 |
| 27 | Nassau County, NY | 1,351,625 | 1,334,544 | 1.3 |
| 28 | Cuyahoga County, OH | 1,283,925 | 1,393,978 | -7.9 |
| 29 | Palm Beach County, FL | 1,265,293 | 1,131,184 | 11.9 |
| 30 | Allegheny County, PA | 1,215,103 | 1,281,666 | -5.2 |
| 31 | Oakland County, MI | 1,202,174 | 1,194,156 | 0.7 |
| 32 | Hillsborough County, FL | 1,180,784 | 998,948 | 18.2 |
| 33 | Hennepin County, MN | 1,140,988 | 1,116,200 | 2.2 |
| 34 | Franklin County, OH | 1,129,067 | 1,068,978 | 5.6 |
| 35 | Orange County, FL | 1,072,801 | 896,344 | 19.7 |
| 36 | Contra Costa County, CA | 1,029,703 | 948,816 | 8.5 |
| 37 | Salt Lake County, UT | 1,022,651 | 898,387 | 13.8 |
| 38 | Fairfax County, VA | 1,015,302 | 969,749 | 4.7 |
| 39 | Fulton County, GA | 1,014,932 | 816,006 | 24.4 |
| 40 | Pima County, AZ | 1,012,018 | 843,746 | 19.9 |
| 41 | Travis County, TX | 998,543 | 812,280 | 22.9 |
| 42 | St. Louis County, MO | 991,830 | 1,016,315 | -2.4 |
| 43 | Westchester County, NY | 953,943 | 923,459 | 3.3 |
| 44 | Milwaukee County, WI | 953,328 | 940,164 | 1.4 |
| 45 | Montgomery County, MD | 950,680 | 873,341 | 8.9 |
| 46 | DuPage County, IL | 930,528 | 904,161 | 2.9 |
| 47 | Pinellas County, FL | 910,260 | 921,482 | -1.2 |
| 48 | Erie County, NY | 909,845 | 950,265 | -4.3 |
| 49 | Fresno County, CA | 909,153 | 799,407 | 13.7 |
| 50 | Shelby County, TN | 906,825 | 897,472 | 1.0 |
| 51 | Honolulu County, HI | 905,034 | 876,156 | 3.3 |
| 52 | Fairfield County, CT | 895,030 | 882,567 | 1.4 |
| 53 | Bergen County, NJ | 894,840 | 884,118 | 1.2 |
| 54 | Mecklenburg County, NC | 890,515 | 695,454 | 28.0 |
| 55 | Marion County, IN | 880,380 | 860,454 | 2.3 |
| 56 | Hartford County, CT | 877,312 | 857,183 | 2.3 |
| 57 | Wake County, NC | 866,410 | 627,846 | 38.0 |
| 58 | Hamilton County, OH | 851,494 | 845,303 | 0.7 |
| 59 | Duval County, FL | 850,962 | 778,879 | 9.3 |
| 60 | New Haven County, CT | 846,101 | 824,008 | 2.7 |
| 61 | Macomb County, MI | 830,663 | 788,149 | 5.4 |
| 62 | Prince George's County, MD | 820,852 | 801,515 | 2.4 |
| 63 | San Francisco County, CA | 808,976 | 776,733 | 4.2 |
| 64 | Kern County, CA | 800,458 | 661,645 | 21.0 |
| 65 | Ventura County, CA | 797,740 | 753,197 | 5.9 |
| 66 | Gwinnett County, GA | 789,499 | 588,448 | 34.2 |
| 67 | Middlesex County, NJ | 789,102 | 750,162 | 5.2 |
| 68 | Pierce County, WA | 785,639 | 700,820 | 12.1 |
| 69 | Baltimore County, MD | 785,618 | 754,292 | 4.2 |
| 70 | Worcester County, MA | 783,806 | 750,963 | 4.4 |
| 71 | Montgomery County, PA | 778,048 | 750,097 | 3.7 |
| 72 | Essex County, NJ | 770,675 | 793,633 | -2.9 |
| 73 | Collin County, TX | 762,010 | 491,675 | 55.0 |
| 74 | El Paso County, TX | 742,062 | 679,622 | 9.2 |
| 75 | DeKalb County, GA | 739,956 | 665,865 | 11.1 |
| 76 | Essex County, MA | 736,457 | 723,419 | 1.8 |
| 77 | Monroe County, NY | 732,762 | 735,343 | -0.4 |
| 78 | Suffolk County, MA | 732,684 | 689,807 | 6.2 |
| 79 | Hidalgo County, TX | 726,604 | 569,463 | 27.6 |
| 80 | Multnomah County, OR | 714,567 | 660,486 | 8.2 |
| 81 | Jefferson County, KY | 713,877 | 693,604 | 2.9 |
| 82 | San Mateo County, CA | 712,690 | 707,161 | 0.8 |
| 83 | Lake County, IL | 712,453 | 644,356 | 10.6 |
| 84 | Oklahoma County, OK | 706,617 | 660,448 | 7.0 |
| 85 | Cobb County, GA | 698,158 | 607,751 | 14.9 |
| 86 | Snohomish County, WA | 683,655 | 606,024 | 12.8 |
| 87 | Will County, IL | 681,097 | 502,266 | 35.6 |
| 88 | San Joaquin County, CA | 672,388 | 563,598 | 19.3 |
| 89 | Jackson County, MO | 668,417 | 654,880 | 2.1 |
| 90 | Norfolk County, MA | 659,909 | 650,308 | 1.5 |
| 91 | Jefferson County, AL | 659,503 | 662,047 | -0.4 |
| 92 | Monmouth County, NJ | 642,448 | 615,301 | 4.4 |
| 93 | Baltimore city, MD | 636,919 | 651,154 | -2.2 |
| 94 | Denton County, TX | 636,557 | 432,976 | 47.0 |
| 95 | Bernalillo County, NM | 635,139 | 556,678 | 14.1 |
| 96 | Providence County, RI | 626,150 | 621,602 | 0.7 |
| 97 | Davidson County, TN | 626,144 | 569,891 | 9.9 |
| 98 | Bucks County, PA | 621,643 | 597,635 | 4.0 |
| 99 | Kent County, MI | 605,213 | 574,335 | 5.4 |
| 100 | Denver County, CO | 598,707 | 554,636 | 7.9 |
Source: U.S. Census Bureau, Population Division
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