NEW YORK (CNN/Money) - Millionaires showed concern about the investment outlook in September, despite being traditionally more optimistic than other affluent investors, according to a Spectrem Group survey.

Spectrem's Millionaire Index fell to an all-time low of 5 in September, indicating that investors were "neutral" about their investment prospects. The August index was 11, with a 2005 high of 21 ("mildly bullish") in February.

The survey consisted of 250 households with investable assets of more than $500,000, with millionaires making up about 100 of the respondents. Spectrem began monthly surveys of affluent investors' attitudes in February 2004.

Asked to state what they believed was the most serious threat to their financial goals , 15 percent indicated the state of the economy, 8 percent said gas and oil prices, 8 percent said the outcome of last year's presidential elections, 6 percent said unemployment, and 6 percent mentioned health-related concerns.

Respondents did not rank Hurricanes Katrina and Rita high among their concerns, indicating that they believed these were short-term issues for the economy. Seventy-one percent said the Gulf Coast disasters would have little or no impact on their future investments.

"Millionaires finally have succumbed to the pessimism that has dogged the overall affluent population throughout 2005," said George H. Walper Jr., president of Spectrem Group, in a statement. "Their focus on longer-term issues, such as the economy, presidential politics and oil and gas prices suggests this pessimism may be with us for some time."

"I believe what we're seeing now is the result of so much bad news: the two hurricanes, the economy, energy prices, the continuing problems in Iraq," said Walper. "That drives investors to be cautious. But at the same time we're not seeing a huge move toward cash, but continued pessimism could cause that."

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The number of millionaires hit a record high this year -- reaching 8.9 million -- read more here.

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