U.S. exchange-traded funds (ETFs) are on a roll so far in 2011, according to State Street Global Advisors, which released its 2011 mid-year ETF outlook report on Thursday.

The report revealed that ETF inflows are set to exceed $100 billion in 2011, a signal of strength in the ETF industry despite historically low interest rates, persistently high unemployment, and global economic uncertainty. U.S. ETFs raked in $56.3 billion in new inflows during the first six months of 2011, an increase of 50.9% over the first half of 2010 and on pace to surpass last year’s total of $111.5 billion. ETF investors poured money into fixed-income, developed international markets, and dividend strategies and moved away from emerging markets and small cap equities during this period.

“With demand for income and non-correlated assets on the rise, a growing universe of professional and retail investors are using ETFs to access precise sources of return and improve the diversification of their portfolios,” said Kevin Quigg, global head of the SPDR ETF Capital Markets Group at State Street Global Advisors, in a press release.  “If flows remain on their current pace, 2011 will mark the fifth consecutive year that ETFs attract more than $100 billion in positive cash flows.”

Although there has been a lot of uncertainty in the stock market, U.S. companies have seen revenues rise, profit margins grow, and are now trading at relatively attractive prices, according to the report.

Three categories of the ETF market that have led the pack are: fixed income, developed international and dividend/ fundamental with gains of $16.3 billion, $12.6 billion and $6.4 billion, respectively. Meanwhile, emerging markets, U.S. small cap and commodity sectors led outflows with $3.7 billion, $2.2 billion and $2.1 billion, respectively.

In addition, U.S. mid cap equities provided strong returns of 8.6%, while small caps returned 7.5% and large caps added 6%.

After all the hype about emerging market stocks over the last two years, they returned only 0.9% during the first six months of the year.

Bonds performed well in the low interest rate environment, especially bonds linked to inflation, which had returns of 8.1%, and international bonds, with returns of almost 6%. Both benefited from increased consumer prices in the U.S. and globally and the relative weakness in the U.S. dollar in the first half of 2011. High yield bonds jumped 4.8%.