A greatest-hits album once stood as a watershed, a milestone chronicling a collection of top-rated hits, culturally significant songs or the end of a stellar career.

But a flood of "best of" titles from acts with only a few years in the business -- and performers with even fewer hits -- has called into question how great a greatest-hits collection is.

Last fall, Britney Spears released "Greatest Hits: My Prerogative," a retrospective of her multi-platinum recording career -- all six years of it. The Backstreet Boys put out "The Hits: Chapter One" in 2001 -- just four years after releasing their first album.

"I don't know what you'd put on a record if you've only been making records for five years. I don't know what those greatest hits would be," says veteran rocker John Mellencamp.

At least Spears and the Boys had a steady collection of hits, or can say they have more than just one.

Among the more questionable collections that have popped up in recent years: "Toy Soldiers: The Best of Martika" (who had one hit from the 1980s); "The Best of Mandy Moore," who had more success as an actress than she ever did as a singer; and "The Best of O.D.B.," from the late rapper who will best be remembered for his wild behavior and legal troubles than his hits (or lack thereof).

And although teen queen Hilary Duff has sold millions of albums, she has released only two discs -- the first one in 2003 -- and neither spawned a Top 10 hit. Her "best of" collection, "Most Wanted," hit record stores Tuesday.

"I guess with music becoming so disposable, things just happen so much faster now," says Collective Soul guitarist Dean Roland, whose band put out a greatest-hits album in 2001. "The short answer to the whole thing is, it comes out to a money issue. The labels can put a greatest-hits album out and it's going to sell."

They can be wildly popular. The Eagles' "Their Greatest Hits: 1971-1975" is the best-selling album in U.S. history, selling more than 28 million copies. They're also cheap to produce and promote.

Sheryl Crow, whose "The Very Best of Sheryl Crow" was a multi-platinum hit in 2003, agrees that in some cases, thoughts of profits weigh heavily. "Part of that is record labels continually try to work their catalogue so they always have money coming in," Crow says.

But Kevin Gore, executive vice president of sales and marketing at Rhino Entertainment, which puts out various greatest-hits packages, compilations and boxed sets, says a "Best of Sugar Ray" -- which Rhino recently released -- is valid, even if some may sneer.

"At the end of the day, there's a larger audience that will buy a greatest-hits collection than might buy the individual albums just because they've chosen not to buy the original albums," Gore says. "Let's say there's two hit songs on a record from five years ago, and then there's another two hit songs from a record that's three years ago. You have a greater opportunity to bring in a larger audience because you're putting all the hits in one place."

It's an idea the Backstreet Boys agree with, though they balked at the idea of putting one out.

"For me as an artist, like, when I see somebody put out a greatest-hits record, they're either finished, or they need some time," says Brian Littrell. "I kind of felt it was too soon to put those great songs on a CD. I think I wanted our fans to miss them a little bit."

However, Howie Dorough says the greatest-hits album -- which sold more than 1 million copies and contained a few new tunes -- may have drawn the casual fan who had never brought a Backstreet Boys CD.

A more critical issue, record companies say, is the decreasing shelf space to carry an artist's catalogue. With the advent of Wal-Mart and Target as major record stores, there are fewer outlets where you might find several different albums from one artist.

"Perhaps songs that were included on albums one, three and five might no longer get the kind of visibility or shelf space that a hits record will provide those particular songs," says Jeff Jones, executive vice president of Sony BMG's catalogue division Legacy Recordings, whose catalogue releases this year range from Miles Davis to Bob Dylan to . . . Martika.

Britney Spears has already put out "Greatest Hits: My Prerogative."