The index rose to 102.8, up for the third consecutive month the Conference Board said.
The index assigns a base value of 100 to the year 2004. A number above 100 shows the indicators have risen compared with the average for the index in 2004. Conversely, if the index is less than 100, the indicators are lower than the average for 2004.
The Conference Board said the Coincident Index for Britain, which measures current economic activity, remained unchanged in August and September after climbing 0.4 percent in July. The Coincident Index is steady at 103.8.
Five of seven component measures in the leading index rose in September but economist Brian Schaitkin said the gains may taper off soon.
"Despite this moderate gain, the economy, which was buoyed by temporary factors like the London Olympics, is likely to slow in the final quarter of this year," said Schaitkin, the European economist at the Conference Board.
"Consumer and business expectations regarding future economic conditions have improved from a few months ago, but remain weak by historical standards. In addition, the Coincident Economic Index for the U.K., which measures current economic conditions, has also slowed since the first half of 2012," he said in a statement.