NEW YORK ( TheStreet) -- Deep-value funds struggled last year.Among the worst performers was Third Avenue Value ( TAVFX), which lost 20.7% and lagged 96% of peers in the world stock category, according to Morningstar. Other funds that finished in the bottom half of their categories include Oakmark International ( OAKIX), Vanguard Windsor ( VWNDX) and Mutual Shares ( TESIX). Seeing the bad results, investors are dumping deep-value funds. But that could be a mistake. For patient investors, deep-value funds can be intriguing holdings that help to diversify portfolios. The deep-value funds buy unloved stocks that sell at big discounts. The portfolios of the deep-value funds tend to be even cheaper than the holdings of typical value funds. While the average large value fund has a P/E of 11.64, Vanguard Windsor has a multiple of 9.96. While deep-value stocks can deliver strong long-terms returns, they tend to take investors on rough rides, sinking hard during downturns and soaring in bull markets. During the past year, deep-value stocks were poor performers as investors fled shaky financials and fled to the safety of rock-solid blue chips. When markets sank in 2008, deep-value funds were crushed. Many shareholders sold near the bottom, but that was a bad decision because the funds soared in 2009 when the markets rallied. "The deep-value stocks were hammered, but they bounced back because most of the companies survived the financial crisis and did better than investors expected," says Russel Kinnel, Morningstar's director of mutual fund research. One of the funds that suffered big losses was Vanguard Windsor. Hurt by troubled financial holdings, the fund lost 41.1% in 2008 and lagged 81% of its peers. With investors fleeing the fund, total assets dropped from $14.7 billion in 2006 to $6.6 billion in 2008. Then in 2009, the fund gained 34.7%, outdoing the S&P 500 by 8 percentage points and topping 91% of its large value peers. To take advantage of deep-value funds, investors must be prepared to hold a fund for years. While many investors buy when the funds are hot, the best time to try a deep-value fund is after it has recorded a cold streak. Now that many of the funds have delivered weak 12-month returns, this year could be a time to try the deep-value approach.