Investors' confidence in the U.S. economy fell in September for a fourth straight month to its lowest level in 13 months on continued concerns about the housing market and elevated energy costs, a survey showed on Monday.

The UBS/Gallup Index of Investor Optimism fell to 68 in September, down 5 points from 73 in August The study for the index was conducted prior to the Federal Reserve's rate cut on 18. I don't believe that the Fed cut is going to solve the housing problem right now, but it is a start, said Maury Harris, Chief US Economist, UBS Investment Bank in a statement. The cuts are going to have to go a bit deeper before we see any real stability.

Recent market turmoil has prompted more and more investors to review their personal holdings, with 34 percent re-evaluating their portfolios, said UBS. Of those who have made a re-evaluation, 58 percent are holding more cash or cash equivalents, 43 percent have reduced their investments in complicated financial instruments such as mortgage-backed securities, and 20 percent have reduced their investments outside the United States.

The Economic Dimension of the Index, which measures investors' feelings about the US economy, fell 7 points to minus 2 in September.

Real estate and housing continue to worry investors with 55 percent believing that the potential for a housing or real estate crash in some local markets is hurting the investment climate a lot. It was the highest level of concern about housing since tracking of this issue began in Oct 2005, UBS said.

The Personal Dimension of the Index, which measures investors' optimism about their individual investment portfolios, rose by 2 points in September to 70.

Investor sentiment about higher energy prices remained essentially unchanged in Sept with 65 percent of investors saying they believe energy prices are hurting the current investment climate compared with 64 percent in August .