Over 17% of US children aged 12-19 years are obese, leading to new issues in differentiating weight status among severely obese adolescents. Using the CDC 2000 growth curves and published equations, this study highlights the pitfalls of using body mass index (BMI) Z-score as an outcome measure in clinical research involving severely obese adolescents. Above BMIs of 40, which are typical for adolescent treatment programs, a wide range of BMI translates to a very narrow range of BMI Z-scores, and BMI Z-scores exhibit an upper limit similar to BMI percentiles. At this level, the correspondence between BMI and BMI Z-score differs by age, sex and starting BMI. Thus, a stable high BMI during adolescence results in increasing BMI Z-scores for boys and decreasing BMI Z-scores in girls. A new supplemental BMI reference may be needed specifically for severely obese adolescents to improve measurement and evaluation of treatment success in this group.