| 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 |
| Warren County | 43 863 | 45 050 | - 1 187 | - 2.6 |
| Bear Lake borough | 193 | 193 | 0 | 0.0 |
| Brokenstraw township | 2 068 | 1 962 | 106 | 5.4 |
| Cherry Grove township | 228 | 155 | 73 | 47.1 |
| Clarendon borough | 564 | 650 | - 86 | - 13.2 |
| Columbus township | 1 741 | 1 776 | - 35 | - 2.0 |
| Conewango township | 3 915 | 4 475 | - 560 | - 12.5 |
| Deerfield township | 333 | 274 | 59 | 21.5 |
| Eldred township | 709 | 669 | 40 | 6.0 |
| Elk township | 551 | 541 | 10 | 1.8 |
| Farmington township | 1 353 | 1 287 | 66 | 5.1 |
| Freehold township | 1 402 | 1 318 | 84 | 6.4 |
| Glade township | 2 319 | 2 372 | - 53 | - 2.2 |
| Limestone township | 418 | 359 | 59 | 16.4 |
| Mead township | 1 555 | 1 579 | - 24 | - 1.5 |
| Pine Grove township | 2 712 | 2 756 | - 44 | - 1.6 |
| Pittsfield township | 1 519 | 1 543 | - 24 | - 1.6 |
| Pleasant township | 2 528 | 2 663 | - 135 | - 5.1 |
| Sheffield township | 2 346 | 2 382 | - 36 | - 1.5 |
| Southwest township | 561 | 626 | - 65 | - 10.4 |
| Spring Creek township | 872 | 843 | 29 | 3.4 |
| Sugar Grove township | 613 | 604 | 9 | 1.5 |
| Sugar Grove borough | 1 870 | 1 745 | 125 | 7.2 |
| Tidioute borough | 792 | 791 | 1 | 0.1 |
| Triumph township | 286 | 314 | - 28 | - 8.9 |
| Warren city | 10 259 | 11 122 | - 863 | - 7.8 |
| Watson township | 322 | 276 | 46 | 16.7 |
| Youngsville borough | 1 834 | 1 775 | 59 | 3.3 |
| Source: U.S. Census Bureau | Prepared by: Pennsylvania State Data Center | |||