People who did the diet program for a year lost twice as much weight as people who just got advice from their doctors.
The research results are in The Lancet medical journal.
Researchers say the study is significant because it is considered the first controlled trial of its kind to directly compare a commercial diet program with standard weight loss guidelines given by family physicians.
It is important to note the study was funded by Weight Watchers. But authors say the company had nothing to do with the study's design, implementation or data analysis.