"E-sports is definitely our fastest area of growth," Phil Hudson of Pinnacle Sports, an online betting site, told NBC News. "It's quite big in Asia and Eastern Europe, but e-sports is really a phenomenon across the world."
It's not nearly as popular as soccer or American football. Still, e-sports have attracted more than 350,000 bets on Pinnacle's website this year, making it the company's eighth most popular sport. (Sorry golf).
Since 2011, the year when Pinnacle began accepting bets on e-sports, the volume of trades placed on video game matches has doubled every year, Hudson said. The company has hired a team of eight e-sports traders responsible for setting lines and wagers.
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Pinnacle isn't the only one. Datbet and E-Sport Bets also take bets from all over the world — although none officially from the United States.
Betting on any sport, including e-sports, is illegal in the United States, except for in some capacity in Nevada, Delaware, Oregon, Montana and, pending a legal battle, New Jersey.
That explains why most sports betting sites are based in places like Curaçao, where Pinnacle is officially headquartered. Hudson, from the company's marketing office in London, told NBC News that he wanted to be "explicitly clear" that Pinnacle Sports does not take "bets from the U.S."
Still, it's clear that a lot of people are taking betting on e-sports, most often on "Dota 2" and "League of Legends" matches.
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Where there is gambling, there is the temptation for players to get a cut of the action. The stakes are relatively small in e-sports — the five-player team that won the 2014 "League of Legends" championship took home $1 million in prize money, or less than soccer superstar Ronaldo gets paid in a single game.
So, while pro e-sports leagues don't have to worry about $15 billion international match-fixing scandals, it's still a concern.
In March, "League of Legends" player Cheon "Promise" Min-Ki reportedly attempted suicide after being implicated in a match-fixing scandal. In 2010, the BBC confirmed that complaints of match-fixing in "StarCraft II" had been reported to Seoul police.
That is bad news for a sport that is trying to establish itself.
"It could devastate a sport," I. Nelson Rose, an expert on gambling law at Whittier Law School, told NBC News. "It could prevent it, or at least delay it by years, from being generally accepted."
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"But, having said that, once the sport is established, I think the world changes."
The Electronic Sports League (ESL) wants to become established. It runs competitions across the globe in "League of Legends," "StarCraft II," "Counter-Strike" and more.
(Riot Games, who held the previously mentioned "League of Legends" championship in October, did not respond to interview requests from NBC News).