Researchers at the University of California, San Diego and the University of Minnesota looked at 80 parents, all of whom had a child aged eight to 12 years who needed to lose weight.
They found that a parent's own weight change played a key role in the likelihood of their child successfully losing weight.
Over the course of the five-month study period, change in parent body mass index (BMI) was found to be the only significant predictor of a child's own weight loss.
Dr Kerri Boutelle, associate professor of paediatrics and psychiatry at the University of California, San Diego, explained: "We looked at things such as parenting skills and styles, or changing the home food environment, and how they impacted a child's weight.
"The number one way in which parents can help an obese child lose weight? Lose weight themselves. In this study, it was the most important predictor of child weight loss."
The findings, which are published in Obesity journal, could have important implications in England, where the latest NHS figures suggest that 31 per cent of two to 15-year-old boys and 29 per cent of girls are obese.