Decades of white flight transformed America's cities. That era is drawing to a close.

In Washington, a historically black church is trying to attract white members to survive. Atlanta's next mayoral race is expected to feature the first competitive white candidate since the 1980s. San Francisco has lost so many African-Americans that Mayor Gavin Newsom created an "African-American Out-Migration Task Force and Advisory Committee" to help retain black residents.

"The city is experiencing growth, yet we're losing African-American families disproportionately," Mr. Newsom says. When that happens, "we lose part of our soul."

For much of the 20th century, the proportion of whites shrank in most U.S. cities. In recent years the decline has slowed considerably -- and in some significant cases has reversed. Between 2000 and 2006, eight of the 50 largest cities, including Boston, Seattle and San Francisco, saw the proportion of whites increase, according to Census figures. The previous decade, only three cities saw increases.

The changing racial mix is stirring up quarrels over class and culture. Beloved institutions in traditionally black communities -- minority-owned restaurants, book stores -- are losing the customers who supported them for decades. As neighborhoods grow more multicultural, conflicts over home prices, taxes and education are opening a new chapter in American race relations.