After several brutal years in the corner office, CEO job security is starting to improve.

Through the just-ended first half of 2007, 27 CEOs at Standard & Poor's 500 companies quit, retired or were fired, says executive search firm Spencer Stuart. That's on pace for an annual rate of 54, or about 10 percent, compared with 72 (14 percent) CEOs replaced in 2006.

Among the Fortune 100 representing the largest companies, the departure rate so far in 2007 also is 10 percent, but it is an even more drastic improvement from the 18 percent who left last year and 14 percent in 2005, says executive search firm CTPartners.

The dropoff this year is the start of a long-term trend of greater CEO job security at companies of all sizes, says Brian Sullivan, CEO of CTPartners. The rash of corporate scandals starting in 2001 led to tough legislation and made boards of directors more powerful and less patient of poor performance. But now those same directors are rethinking the short-term pressure on CEOs to meet quarterly earnings at the expense of long-term strategy, Sullivan says.