U.S. companies are buying back the most stock in four years, taking advantage of record-high cash levels and low interest rates to purchase equities at valuations 15 percent cheaper than when the credit crisis began.

Corporations have authorized more than $453 billion in repurchases this year, putting 2011 on track for the third- highest annual total behind 2006 and 2007, data compiled by Birinyi Associates Inc. show. Warren Buffett's Berkshire Hathaway Inc. bought shares for the first time, and Amgen Inc. sold debt to fund its buyback. U.S. companies spent 70 percent more on their stock last quarter than a year ago, according to financial filings as of Nov. 11.

Market bulls say the rise shows executives are confident the U.S. economy will avoid a recession. While the Standard & Poor's 500 Index peaked the last time buybacks were this high, companies in the gauge are generating three times as much cash, price-earnings ratios are lower and 10-year Treasury yields are around 2 percent, data compiled by Bloomberg show. Bears say the increase means companies lack better uses for capital.