When Fox Sports’ N.F.L. guru Jay Glazer gets a call or text message from boxer Chuck Liddell, he knows pro football season is fast approaching. Liddell, gridiron-sized at 6-foot-2 and 205 pounds, wants what millions of fantasy-football mavens desire—any tidbit of information that can be used on draft day, when mere mortals select running backs, quarterbacks, wide receivers, and kickers for fictional squads that use real statistics.

Football is by far the most-watched sport in the U.S., and its pinnacle event, the Super Bowl, is viewed in more than 40 percent of the 111 million U.S. households with televisions. But while pro football may already have a strong audience, its fantasy version is growing faster than grass on the field after a summer rain.

In fantasy football, participants, or “owners,” form leagues and select real N.F.L. players for their teams, which accrue points based on those players’ actual game performances. (Wilfred Winkenbach, a part-owner of the Oakland Raiders who is credited with inventing fantasy football, came up with the point system in 1962 with the help of two sportswriters.) Depending on the format, a league’s champion is either the team with the most total points or the one that emerges victorious after a series of head-to-head matches.

As of August 2006, about 12.8 million American adults had played fantasy football, making up about 80 percent of the $4 billion fantasy sports market, according to the Fantasy Sports Trade Association. (About 4.8 million people play fantasy baseball, F.S.T.A. says.) Industry experts say the number of players has roughly doubled since 2000.

“We’re amazed at how quickly fantasy football has evolved, from the niche it was in during the early-1990s-to-2000 period to where it is now,” says 18-year fantasy-football veteran Jeff Ma, president and founder of Pro Trade, a San Mateo, California, company that lets fans trade on a sports stock market on its website. “The clearest sign is how much major media outlets cover the fantasy side of the N.F.L.”

Fantasy football has so many devotees that the television networks that broadcast N.F.L. games—Fox, CBS, NBC, and ESPN—have altered telecasts to accommodate fantasy players. They have created shows devoted to fantasy draft strategy, in which experts such as Glazer suggest draft orders, mention sleeper picks, and emphasize which players to avoid. The networks have also incorporated bottom-of-the-screen scrolls during games to keep viewers abreast on statistics from around the league, and commentators frequently mention players’ individual achievements throughout the day’s telecasts. Football fans may be interested in general, but fantasy players take special note because the outcome of their fantasy matchups can literally come down to one completed pass, fumble, or missed field goal attempt.

The television push extends to the websites that run fantasy leagues. Not only do the N.F.L. and Yahoo have fantasy setups online, but so do ESPN, CBS Sports, and Fox Sports. Since their revenue comes from ads, these sites are free to fantasy players, who often augment their fantasy analysis using websites such as Glazer’s footballinjuries.com, a subscription service for pro-football news and injury reports that has grown more than sixfold in the past five years.

“Fantasy football is the gorilla,” says Glazer. “The devoted fan is hooked, and now the marginal fan is much more involved. To me that spells limitless potential for this business.”

According to F.T.S.A. president Jeff Thomas, there are three main reasons for fantasy football’s growth: the appeal of delving into football stats, plain love of the sport, and prize money—league winners usually win cash. Also, fantasy football can bring extra drama and excitement to watching your team on Sunday afternoons (and Monday nights), not to mention something extra to talk about at the office.

On the plus side for the N.F.L. and the TV networks: Fantasy football players have a stake in games that might otherwise have little meaning to them. This generates more buzz for each matchup, so a New York Jets fan might end up caring deeply about a contest between the Seattle Seahawks and Jacksonville Jaguars.

“Players really don’t have team loyalty anymore, so why should the fan?” says Dave Cella, president of Payday Sports, a Wilmington, Delaware, company that conducts fantasy drafts and leagues with entry fees ranging from $25 to $10,000 a team. He says his business is growing by leaps and bounds. “If anything, playing fantasy football brings you closer to the action because you literally care about every single game.”

Who’s playing fantasy football? According to F.S.T.A., 92 percent of players are male professionals who spend at least three hours a week managing their teams. About 77 percent are married with annual incomes of more than $76,000. More than half started playing offline, says the organization, which has funded market research into the phenomenon for the past five years.

Even women, long on the football-watching sidelines, are starting to find the fantasy version appealing.

“I love it competitively and as a way to follow the game much more closely,” says Katie Hofmann, a 23-year-old law student and part of a 12-team league last season at the New York law firm Dewey Ballantine. “Fantasy football makes every Sunday an event.”

Glazer, the frenetic football aficionado with the ever-buzzing cell phone, is convinced that the fantasy frenzy is still early in the first quarter.

“The interest is endless. I have restaurant managers calling every week asking who is hurt and who isn’t,” he says. “There isn’t that much I can tell them. But I better get a reservation when I need it.”

Fox Sports chairman David Hill agrees that the fantasy football angle has to be considered during an N.F.L. game, but only up to a point. “My sense is the fantasy element of our telecasts has gone as far as it can,” he says. “You don’t want to alienate the true fan.