The mountain of evidence connecting professional football and long-term brain damage grew this week with publication of a new study that examined the brains of former NFL players. Boston University scientists found 110 of the 111 post-mortem brains showed signs of chronic traumatic encephalopathy (CTE), a degenerative brain disease linked to repeated hits to the head. Linemen had it the worst, while punters seemed to escape relatively unharmed.

This kind of brain damage and the NFL’s response—or lack thereof—has dogged the league for the past decade. Football has seen class action lawsuits, congressional hearings, and efforts to make the game safer by banning certain hits and designing new helmets. But neurologists involved in this new study, as well as other experts, say another sport may rival football’s impact on the brain: soccer.

Soccer isn’t usually considered a contact sport, although gameplay hits can result in concussions. “Soccer has repetitive impacts, from player to player and players heading the ball,” says Boston University’s Ann McKee, an author of this week’s study of NFL players. “It doesn’t matter how you do it, just that you do it and do it repetitively.” And with 265 million players worldwide, soccer represents a potentially huge pool of head injury patients.

One US pro soccer team, DC United, lost six players to retirement over the past decade due to concussions, while another two players missed playing time this season with head injuries. In a recent lawsuit against the team and its coach, former DC United goalkeeper Charlie Horton said one of his teammates elbowed his head on purpose in 2016, giving him a concussion and ending his professional career. And in 2015, the US Soccer Federation, the sport’s governing body, settled a proposed class action lawsuit by limiting heading by youth soccer players.

Lawsuits aren’t scientific evidence, but an increase indicates that players are concerned about how their sport responds to concussions and the possible long-term risk of brain disease. Case studies have shown that retired professional English and Brazilian soccer players with a history of concussions can later show signs of dementia; autopsies revealed CTE-riddled brains. In April, BU researchers reported on the case of a former American high school soccer player who had 19 soccer-related concussions, a history of depression and mental health problems who died aged 24 of a drug overdose. An autopsy revealed that he also had CTE, according to a report in the journal Neurology.

Those case studies have limited scientific value, of course; to nail down the connection between soccer and brain damage, the sport will need bigger sample sizes. The big difference between soccer and football is the number of former NFL players and their families who have come forward asking for help from the medical community. McKee’s group in Boston has set up a “brain bank” where families of ex-football players (pro and college athletes) who are worried about their mental state can donate their brains for research. So far they have received 425 brains; CTE has been found in 270 of them. That kind of focused research effort hasn’t yet been developed for soccer.

Even in the case of football-related brain injuries, there’s no absolute proof that concussions cause CTE. In fact, scientists say that it may be the less powerful “sub-concussive” hits that both football and soccer players receive all the time that could trigger the disease. But at New York’s Albert Einstein College of Medicine, neuroscientist Michael Lipton has been working to identify the trigger. “In soccer, where you have people repeatedly hitting their head over time,” says Lipton, “the question is how much does it take to lead to a pathology that rises to a level where there are functional effects.”