The participants said they were gathered at the Vaxjo clinic Wednesday night to measure the amount of weight they had lost with the help of the program when they heard a loud noise shortly followed by the collapse of the floor, Swedish news agency Tidningarnas Telegrambyra reported Thursday.
"We suddenly heard a huge thud; we almost thought it was an earthquake and everything flew up in the air. The floor collapsed in one corner of the room and along the walls," one of the members said.
The group said the floor then gave out in other parts of the room and the smell of sewage spread throughout the area.
"We're going to have to find a replacement premises," Weight Watchers consultant Therese Levin said.
The participants, who were not injured, said they finished weighing one another in a nearby hallway.
Weight Watchers said the cause of the collapse is being investigated.