The board's index of leading indicators fell 0.2 percent in March after retreating a revised 0.5 percent a month earlier. The decline matched expectations of Wall Street economists.
Five of the 10 leading indicators fell in March: building permits, jobless claims, interest rate spreads, money supply and consumer expectations.
The weakness in the leading index is worrisome, "but the information available so far in April suggests that these declines will not continue," the board said.
Consumer confidence has been buoyed since the war in Iraq began.
The Conference Board's index, designed to forecast economic trends three to six months in advance, has been relatively flat since December 2001.
"The flatness in the leading index suggests that U.S. real gross domestic product growth will stay in the 2 percent to 3 percent range for now," the board said. "As long as economic growth is constrained in this range, the labor market cannot improve."
"In addition to the intensified nervousness over oil prices, war and the potential of a terror attack, it is the more fundamental plummeting of consumer expectations that raises the specter of a fall-off in consumption growth," said Ken Goldstein, economist at the board.
"The combination of the slowing in consumption growth and the delayed start to more investment has effectively extended the soft spot that the economy has been in," Goldstein said.