This item has been corrected.

In October, some 15 million people tuned in to watch Major League Baseball’s World Series in the United States. But that’s nothing compared to the other big sporting tournament that took place around the same time: In late September and early October, 32 million people watched the League of Legends Season 3 World Championship, according to a new report (pdf) from SuperData, a games research company.

Indeed, the finals of the competitive tournament for players of the online multiplayer game resembled a major physical-world sporting event, with the 18,000 tickets to watch it live at the Staples Center in Los Angeles selling out in one hour. On the day, the capacity crowd gathered to watch teams do battle against each other as they spawned minions and unlocked glyphs.

Watching other people play computer games—or eSports—is big business. More than 71 million people worldwide watched eSports last year, half in the United States, according to SuperData. And one in five American gamers either watches or participates in eSports. As more people watch, the rewards are getting bigger too. Some $25 million in prize money was given away in competitive gaming championships in 2013. League of Legends alone was worth over $2 million in prize money. Even the US government recognizes eSports as a legitimate sporting activity—at least at its borders. Last year, it agreed to grant professional athlete visas to people traveling to the US to play video games.

Why would someone want to watch other people play video games? Unlike viewers of traditional sports such as football or basketball, the people watching eSports tend to play the game regularly as well. They watch to learn new moves and improve their own game, says SuperData’s CEO, Joost van Dreunen.

That playing experience helps explain why watching eSports is so popular: Multiplayer online games and shooter games take a long time to master, and novice players learn by watching professionals. ”It’s how you learn, and get to see some really cool moments in the game,” says van Dreunen. These sports fans are dedicated too: the average viewing sessions lasts over two hours, and fans tune in to eSports an average of 19 times a month.

Watching other people play computer games is not a new phenomenon. SuperData traces its origins back to the early 1980s, when television stations broadcast Starcade, a game show in which contestants would play arcade games on air. By 2011, there was a big enough market for the establishment of twitch.tv, a online eSports broadcast website with its own sports commentators. In 2013, the website counted an average of 45 million unique visitors every month, who watched a total of 13 billion minutes of gameplay each month.

Correction (Apr. 4, 2014): An earlier version of this article incorrectly said twitch.tv receives 45 million unique visitors every year. In fact, they hit that number every month.