Researchers at McGill University Health Center in Canada compared more than 1,000 morbidly obese people who had weight loss surgery with thousands who did not. The overall risk of cancer was 78 percent lower in the surgery patients.
For breast cancer, it was reduced by 83 percent.
Weight loss surgery has already been proven to lower the risk of diabetes and heart problems.
The study is in the journal Surgery for Obesity and Related Dieases.