Mary Lou Wehrli and her brother Grant use to help them lose 20 pounds each. They post their weight and taunt each other online.
Video: Online betting could help you lose weight
"That motivation has had me get off the couch when I really didn't want to work out because I was thinking, I have to report in three days," said Grant.
Teasing is all part of the fun. The program depends on the honor system.
"This motivation of this younger brother that's going to lose a bet to me, it's just so cool," said Mary Lou.
On another site, , you hand over your credit card number and each week that you don't meet your goal you're charged.
"A user says, 'You know, I want to lose a pound a week for the next 10 weeks. I'm putting $10 on the line each week. Each week I don't lose that pound, I lose the $10," said Jordan Goldberg of SticK.com.
You choose where the money goes - a charity or an anti-charity.
"That's where we take your money and we donate it to an organization that you oppose. So let's say you're pro-gun control, we send your money to the NRA," said Goldberg.
Dietitian Elisa Zied says diet betting can backfire.
"The concerns I have is that if you're not winning the bet, this can lead to unhealthy behaviors, whether it's over exercising or dramatically cutting your calories," she said.
She says set a realistic goal of one to two pounds a week. That's what Mary Lou and Grant are aiming for.
By the way, on StickK.com, you have to appoint a referee to keep it honest.
If you're wondering what the most popular anti-charity is to date, it's the George W. Bush Library, which has received just short of $6,000.