This paper discusses the results of the first study to analyze the effects of fast-food advertising on body composition among youths as measured by percentage body fat (PBF). The study also examined the sensitivity of these effects to measuring youth obesity using body mass index (BMI). Researchers found that exposure to fast-food restaurant advertising on television causes statistically significant increases in PBF in youths. The results are consistent with those obtained by using BMI-based measures of obesity; however there is a larger effect using PBF measures. Results indicated that a complete advertising ban on fast-food restaurants on television would reduce BMI by approximately 2 percent and would reduce PBF by approximately 3 percent.