WASHINGTON (AP) — U.S. consumer confidence shot up to an 18-year high in September.

The Conference Board, a business research organization, says its consumer confidence index climbed to 138.4 in September from 134.7 in August. This month's reading was the highest since September 2000.

The index measures consumers' assessment of current economic conditions and their outlook for the next six months. Both improved in September.

"These historically high confidence levels should continue to support healthy consumer spending, and should be welcome news for retailers as they begin gearing up for the holiday season," said Lynn Franco, the Conference Board's director of economic indicators.

The U.S. economy grew at an annual pace of 4.2 percent from April through July, fastest rate in nearly four years.