The largest public pension fund in the U.S. lost almost $67 billion in 12 months, more than 25 percent of its value, as stock markets tumbled.

The market value of the California Public Employees' Retirement System declined to $193.7 billion as of Oct. 9, from $260.6 billion a year earlier. Between Sept. 15, when Lehman Brothers Holdings Inc. filed for bankruptcy, and last week, the fund's stock holdings declined by $12.4 billion to $36.8 billion, according to data compiled by Bloomberg.