Oprah Winfrey

CHICAGO — Oprah Winfrey will announce today her powerhouse daytime television show, the foundation of a multibillion-dollar media empire with legions of fans, will end its run in 2011 after 25 seasons on the air.

Winfrey was to announce the final date for "The Oprah Winfrey Show" during a live broadcast, according to her production company, Harpo Productions.

Once a local Chicago morning program, the production evolved into television's top-rated talk show for more than two decades, airing in 145 countries worldwide and watched by an estimated 42 million viewers a week in the U.S. alone.

"Oprah Winfrey is in a category of her own," said Robert Thompson, professor of television and popular culture at Syracuse University. "This is a great American story, and like any great American story, it's supersized."

A Harpo spokeswoman declined to comment Thursday on Winfrey's future plans except to say "The Oprah Winfrey Show," which has seen ratings slip 7 percent from a year ago, will not move to cable television.

Winfrey, 55, is widely expected to start up a new talk show on OWN: The Oprah Winfrey Network, a much-delayed joint venture with Discovery Communications that is expected to debut in 2011. OWN is to replace the Discovery Health Channel and will debut in some 74 million homes. An OWN spokeswoman declined to comment Thursday.

CBS Television Distribution, which distributes "The Oprah Winfrey Show" to more than 200 U.S. markets, held out hope it could continue doing business with Winfrey, perhaps producing a new show out of its studios in Los Angeles.

"We know that anything she turns her hand to will be a great success," CBS said in a statement. "We look forward to working with her for the next several years, and hopefully afterwards as well."

Winfrey's 24th season opened this year with a bang, as she drew more than 20,000 fans to Chicago's Magnificent Mile for a block party with the Black Eyed Peas. She followed with a series of blockbuster interviews — Mike Tyson and Evander Holyfield, exclusives with singer Whitney Houston and ESPN's Erin Andrews, and just this week, former Alaska governor, GOP vice presidential candidate Sarah Palin.

As a newcomer, "The Oprah Winfrey Show" chipped away at talk king Phil Donahue's dominance. Later, it turned to inspiration. The show covered a range, from interviews with the world's celebrities to an honest discussion about Winfrey's weight struggles.

"As the show evolved, it really kind of dressed up the neighborhood of the daytime talk show," Thompson said.

The loss of "The Oprah Winfrey Show" would be a blow to CBS, which earns a percentage of hefty licensing fees from TV stations that use it — largely ABC affiliates. CBS Chief Executive Leslie Moonves told analysts two weeks ago the contract with the show runs through most of 2011 and "if there's a negative impact, it wouldn't hit us until '12."