The list of the top ten most populous U.S. cities remained relatively unchanged with the exception of Phoenix, which leapfrogged over Philadelphia in the top five, according to annual data (pdf) released by the U.S. Census Bureau on Thursday. The estimates are based on Census 2000 population counts and updated with information from building permits and other estimates of change.

The report also revealed the country's fastest growing cities (pdf) with North Las Vegas, Nev. topping the list; the largest population increases (pdf) from 2005 to 2006, led by Phoenix; and the largest population decrease (pdf) from 2005 to 2006. New Orleans, still reeling from the aftermath of Hurricane Katrina, suffered the biggest loss - 50 percent of their population.

New York City, the most populous city in the U.S. according to the data, was the only Northeastern city in the top ten with an increase in population. Phoenix's surpassing of Philadelphia is, what the Census Bureau calls, "the latest evidence of a decades-long population shift."

"The trend has been that cities in the Northeast and Midwest tend to be losing (population)," explains Census Bureau demographer Greg Harper. "The fastest growing cities are in the South and West."

The National Association of Home Builders' (NAHB) Bernard Markstein, a senior economist and director of forecasting, says Phoenix's home building activity accelerated rapidly, perhaps too rapidly, to accommodate their population growth.

"Phoenix has been one of the areas where there was a lot of home building during the boom period," Markstein told BUILDER Online. "With the population growth there, there has been some overbuilding in that area and some slowdown."

Some of the cities on the fastest-growing cities list, like McKinney, Texas; Gilbert, Ariz.; Lancaster, Calif.; and Cary, N.C. are not household names. "If you go back and look at these cities 10 to 20 years ago, a lot of them were small - below 100,000 in population," Harper says. "They've grown very fast recently."

Learn more about markets featured in this article: Phoenix, AZ, Philadelphia, PA.