New York ( TheStreet) -- With $31 billion in assets, PowerShares QQQ ( QQQ) ranks as one of the most popular exchange-traded funds. Lately shareholders have had cause to be pleased. The ETF, which tracks the Nasdaq 100, has returned 17.9% this year, outdoing the S&P 500 by more than 3 percentage points, according to Morningstar.The long-term returns have also been notable. During the past 10 years, the PowerShares fund returned 10.8% annually, topping the S&P by 4 percentage points and surpassing 99% of large growth competitors. But after the big rally, there is reason to be wary. The ETF owes much of its stellar showing to Apple ( AAPL), which accounts for 19.7% of the assets. This year Apple shares have returned 49.8%. During the past decade, the stock has returned 54% annually. Besides Apple, the fund has big positions in a handful of technology giants, including Microsoft ( MSFT), Google ( GOOG) and Oracle ( ORCL). Altogether eight technology stocks account for 52% of the assets. If the technology sector disappoints, then the QQQ fund could sink badly. Even if you are bullish on technology, the PowerShares fund may not be the best choice. The problem is that the ETF has a peculiar structure, holding 100 of the biggest stocks that trade on Nasdaq. While the S&P 500 is deliberately designed to provide a cross section of the market, the QQQ index focuses on whatever stocks happen to dominate the Nasdaq listings. The benchmark has 63% of assets in technology, and most of the rest are in health care and consumer cyclicals. There are no energy or financial stocks. As a result, the benchmark does not provide broad market exposure, and it is not a pure play on technology. Launched in the bull market of the late 1990s, the QQQ fund became an immediate hit because it offered a convenient way to bet on Nasdaq's hottest growth stocks. When the Internet bubble burst in 2000, the fund sank hard. Today the ETF remains popular with investors who want to place a bullish bet on the market, says Matt Hougan, senior editor of IndexUniverse.com. "When the market is rising, people reach for the QQQs," he says.