Betty Jarra, an organizer of the event, said the underdressed people on the commuter trains Sunday were taking part in what has become an annual tradition, The Local.se reported Monday.
"It's always been a success since we started with just eight people back in 2009. And it works because it makes people happy, and it gets them to react. It doesn't matter how they react -- as long as they do," Jarra said.
"People get engaged more than they usually do in Stockholm and we think that's important," she said.
The event was inspired by a New York tradition, started in 2002 by performance group Improv Everywhere. There are now No Pants Subway Rides in 60 cities around the world.