| Population Change 1990-2000 | ||||
| County or Municipality | 2000 Population | 1990 Population | Population Change | Percent Change |
| Pennsylvania | 12 281 054 | 11 881 643 | 399 411 | 3.4 |
| Union County | 41 624 | 36 176 | 5 448 | 15.1 |
| Buffalo township | 3 207 | 2 877 | 330 | 11.5 |
| East Buffalo township | 5 730 | 5 245 | 485 | 9.2 |
| Gregg township | 4 687 | 1 114 | 3 573 | 320.7 |
| Hartleton borough | 260 | 246 | 14 | 5.7 |
| Hartley township | 1 714 | 1 896 | - 182 | - 9.6 |
| Kelly township | 4 502 | 4 561 | - 59 | - 1.3 |
| Lewis township | 1 405 | 1 222 | 183 | 15.0 |
| Lewisburg borough | 5 620 | 5 785 | - 165 | - 2.9 |
| Limestone township | 1 572 | 1 346 | 226 | 16.8 |
| Mifflinburg borough | 3 594 | 3 480 | 114 | 3.3 |
| New Berlin borough | 838 | 892 | - 54 | - 6.1 |
| Union township | 1 427 | 1 300 | 127 | 9.8 |
| West Buffalo township | 2 795 | 2 254 | 541 | 24.0 |
| White Deer township | 4 273 | 3 958 | 315 | 8.0 |
| Source: U.S. Census Bureau | Prepared by: Pennsylvania State Data Center | |||