Introduction: The predictive capability of waist circumference (WC) and body mass index (BMI) in cardiometabolic risk is well known. Both indices increase with age, yet the changes in WC relative to BMI, especially for a given BMI category, are not fully understand. This study aimed to assess changes with increasing age, of the ratio of WC-decile to BMI-decile (WC-d/BMI-d), stratified by sex and BMI categories. Hypothesis: WC-d/BMI-d increases with age, with significant differences within sex and BMI categories. Methods: For 29,256 participants (mean age: 47.8 years, 51% women) of the National Health and Nutrition Examination Survey during 2008-2017, WC-d and BMI-d (range 1-10 for both) were defined, and WC-d/BMI-d was calculated. Multivariable linear regression was used to assess the independent association of age with WC-d/BMI-d. The model was adjusted for sex, ethnicity, and comorbidities. The interaction terms age x sex and age x BMI category were incorporated into the model. Separate models were applied for males, females, and BMI categories. Results: In multivariable linear regression, WC-d/BMI-d increased with age among the total study population (0.006, p<0.001); and the interaction terms age x sex and age x BMI-category were significant (p<0.001for both). The increasing trend of WC-d/BMI-d with age was significant for males (0.008, p<0.001), females (0.004, p<0.001), and the BMI groups examined (normgal weight: 0.011, p<0.001, overweight: 0.007, p<0.001, and obesity: 0.002, p<0.001). Greater increases were found among males (Figure 1A) and among individuals with normal BMI level (Figure 1B). Conclusion: The increase in WC-d/BMI-d with age reflects a steeper increase in WC than in BMI, with greater changes among males and among individuals with normal BMI level. These results emphasize the importance of WC measurement in clinical practice, especially among males and individuals with normal BMI. It is worth considering WC-d/BMI-d as an additional measure in obesity management among adults.