It was a finding heard around the sports world. Researchers at Boston University announced in July that they had detected evidence of chronic traumatic encephalopathy (CTE) in 110 donated brains of former NFL players. They had studied 111. The result suffered from selection bias—the brains had been donated because those men had demonstrated symptoms of the degenerative brain disease—but the report’s ironclad takeaway is that the illness is far more prevalent in pro football players, subject to years of repeated hits to the head, than in the general public. The unavoidable conclusion: Football’s concussion problem is far worse than originally thought.
The results highlight an existential crisis for America’s most popular sport. This fall, NFL fans will have to wonder whether the next bone-crunching tackle they see on the gridiron will haunt their favorite player for years to come. And if it seems likely that it will, can they stand to keep watching?
A business empire hinges on the answer to that question. The NFL employs thousands of people and is expected to pull in $14 billion in revenue this year, between ticket sales, merchandising, sponsorships, and massive TV-rights deals. Commissioner Roger Goodell has said he wants that number to reach $25 billion within a decade. College football is also a colossus, with ESPN paying $7.3 billion over 12 years for the rights to televise just seven bowl games a year. But the real economic impact comes from the many ancillary businesses that surround the sport. Fantasy football is estimated to be worth billions annually, and ESPN and other networks rake in billions from subscriptions and advertising. Athletic apparel companies like Nike and Adidas invest millions in licensing and sponsorship agreements with the NFL and NCAA, not to mention the endorsement deals for individual players.
In the near term, industry analysts see few signs of football Armageddon. (Boxing, another sport known for its brutality, saw its popularity wane, but only gradually, over the course of decades.) But there may be some soft spots in the league’s armor. For starters, TV ratings for NFL games dipped 9% last year. While that decline could have various causes, there’s no equivocation over what’s driving a rash of early NFL player retirements. From promising San Francisco 49ers rookie linebacker Chris Borland in 2015, to 26-year-old Baltimore Ravens offensive lineman John Urschel just days after the Boston CTE report, more players are hanging up their cleats over fears of long-term brain damage. In January, Heisman Trophy–winner Bo Jackson told USA Today he would never have played if he’d known the risks, adding, “There’s no way I would ever allow my kids to play football today.”
Indeed, the NFL’s greatest challenges are yet to come. Last year an HBO/Marist poll found that 44% of parents with sons under 18 were less likely to let them play football—meaning the NFL’s talent pipeline (and fan reservoir) could eventually dry up.
The NFL isn’t in denial about the problem. It’s working with youth football organizations on rule changes aimed at limiting head-on collisions, and is spending $1 million at schools to promote flag football, an increasingly popular collision-free version of the sport. NFL officials have publicly admitted the link between football and degenerative diseases like CTE. And Goodell touts the NFL’s efforts to improve player safety, including funding research to curb concussions by upgrading helmets and the field itself. The league has also pledged millions to neuroscience research.
But even a wholesale rethinking of its game play might not fix football’s problem. Fans driven from the sport aren’t guaranteed to return, especially if dramatic rule changes significantly alter the feel of the game. That’s the unsettling paradox of the financial threat looming over America’s massive football economy. It’s a multibillion-dollar gamble no matter what path the sport takes.
A version of this article appears in the Sept. 15, 2017 issue of Fortune with the headline "Football Absorbs a Knockout Blow."