2015 was an atypical year for Warren Buffett and Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B). The S&P 500 was down slightly for the year, and Berkshire declined even more.

Typically, Buffett's stocks outperform the market during years when the market is negative or slightly up, and underperform in years when the market soars. However, several of Buffett's major stock holdings had a particularly tough time in 2015.

For example, Wal-Mart (NYSE:WMT) was Buffett's worst-performing major holding in 2015, with a total return of negative 26.7%. The company faced several headwinds this year, including the announcement of an increased minimum wage companywide, lower profits due to the strong dollar and increased investment in the company's e-commerce efforts, and several quarters of lower-than-expected earnings.

American Express (NYSE:AXP), another major Buffett stock, also had a terrible year in 2015. The company lost its exclusive partnership with Costco and was on the losing end of an important antitrust lawsuit. In addition, thanks to a discouraging quarterly report and overall market weakness, the stock has plunged an additional 20% in the first few weeks of 2016.

It wasn't all bad news and some of Buffett's stocks were winners in 2015, but they weren't enough to overcome the bad performance of some of Berkshire's biggest holdings. Here's a slideshow of how Buffett's 20 largest stock holdings performed in 2015.