The company's shares soared Tuesday after a new commercial starring Winfrey aired where she says she has lost 26 pounds on the plan.
Winfrey bought a 10 per cent stake in the weight-loss company in October. The stock is up more than 90 per cent since then.
Weight Watchers has also made changes to its plans to better fit in with Winfrey's message of living a healthy and happier life, and not just focusing on weight loss.
Shares of New York-based Weight Watchers International Inc. rose almost 20 per cent in afternoon trading. But they are still down about 21 per cent from a year ago.