Forbes reports:

The swimsuit edition became a stand-alone issue for Sports Illustrated in 1997, and has become one of Time Inc.’s biggest revenue drivers over the years, bringing in more than $1 billion. In 2005 alone, the swimsuit issue generated an estimated $35 million in ad sales. On top of that there are outside merchandise sales like calendars, too. Today, it generates about seven percent of SI’s annual revenue.

Henry Blodget adds:

The swimsuit issue accounts for 11% of "Sports" Illustrated's annual revenue, an SI honcho said on Bloomberg last night. We assume he meant ad revenue, although it's possible he was referring to the whole kit and kaboodle.

If we assume 50 yearly issues, each issue represents about 2% of available annual ad inventory. Which means the swimsuit issue is 5X as profitable as the sports the magazine covers in the other 49 issues.

More facts:

In 1997, SI's swimsuit edition received special-issue status. It has since become the single best-selling issue in Time Inc.'s magazine franchise.

In 2005, the swimsuit issue brought in an estimated $35 million in ad sales; Today It generates 7 percent of Sports Illustrated's annual revenue, SI editor Terry McDonnell told USA Today.

The swimsuit issue isn't just a boon to advertisers — bikini and jewelry designers whose items are featured in the special issue also experience a major boost in sales.

Regular Sports Illustrated has more than 3 million subscribers and is read by about 23 million people a week.

The swimsuit issue traditionally sells more than 1 million copies on newstands (about 10 to 15 times as much as regular SI). Last year, consumers forked over $6.99 to get their hands on the popular glossy.

That's a big business. Swimsuits are 5x more profitable than other sports.