The total population in the United States grew by about 10 percent between 1990 and 1999, but the situation varied across the country. The fastest growth occurred in the West, especially among several "Rocky Mountain states": Arizona, Colorado, Idaho, Nevada, and Utah. All of these states grew by more than 20 percent. The population in Nevada increased by 45 percent since 1990. The slowest growth (5 percent change or less) was recorded among states in the Midwest and Northeast. The populations in Connecticut, the District of Colombia, North Dakota, and Rhode Island declined. Of the southern states, Georgia had the biggest gains, but there was also substantial growth in Florida and Texas. In the United States, such short-term, rapid growth is most often caused by migration flows, but birth and death rates also have an impact in some places. Utah, for example, has among the highest fertility rates and longest life expectancies of any state.

These ongoing migration flows cause shifts in the geographic distribution of the U.S. population. In 1999, Illinois surpassed Pennsylvania as the fifth most populous state, and by 2025, Florida is projected to take over as the third largest state, replacing New York. In the case of Florida, migration has fundamentally altered the state's age, racial, and ethnic composition and has put a strain on environmental resources.