By Chris Dieterich 
 
 

NEW YORK--August is poised to see the biggest cash withdrawal from all U.S. exchange-traded funds in more than three years, according to preliminary data from BlackRock Inc. (BLK), the largest U.S. ETF provider.

Some $16.1 billion flowed out of U.S. ETFs in August, through Thursday. Barring some massive moves Friday, the outflow this month would be the worst for the U.S. ETF industry since January 2010, when $17.1 billion came out.

The prime culprit is the ETF market's biggest and most widely traded fund, the $137 billion SPDR S&P 500 (SPY). The ETF had shed a whopping $13 billion alone, through Thursday. Big outflows from stock & bond ETFs overshadowed inflows from international equity ETFs, which took in $5.6 billion.

Questions about when and if the Federal Reserve will reduce its stimulus efforts have dominated the past month, with the threat of U.S. military strikes against Syria fraying nerves earlier this week. The S&P 500 has closed Thursday down 4.2% since hitting its all-time high on Aug. 2, and was on pace for its worst monthly decline since May 2012.

Rising interest rates also shook ETF investors out of bond ETFs, which were poised to see outflows of $6.8 billion. August's outflow was setting up to come in shy of the $7.3 billion that poured out of bond ETFs in June, when Federal Reserve Chairman Ben Bernanke first said that the central bank could begin to dial back its monthly bond buying this year.

Mutual fund data show similar caution in stocks. Stock mutual funds saw their stream of cash slow in August, but still was on pace to see inflows, according to preliminary data from Lipper. Some $10.8 billion came into stock mutual funds in August, compared with an inflow of $22 billion a month earlier.

Bond mutual funds were set to see inflows for the first time since May, taking in $3.8 billion.

"We don't know what's going to happen in Syria, or with the Fed," said Reed Choate, portfolio manager of equities and fixed-income at Neville, Rodie & Shaw in New York, which oversees $1.3 billion. "We're kind of digesting this week and seeing what's going to happen, waiting to make moves until we get more clarity."

Write to Chris Dieterich at christopher.dieterich@dowjones.com

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