Background: Pediatric obesity remains a significant public health issue within the U. S., affecting up to 17% of children. Given ongoing evidence of increased health risks associated with increasing severity of obesity, efforts have been made to better define and characterize this higher risk pediatric cohort. This study examines trends in the degree of obesity by sex and race/ethnicity across the pediatric lifespan to characterize a contemporary cohort of children with obesity receiving care in the community-based setting. Methods: We utilized data from a large and diverse population cohort of 7956 children and adolescents ages 3-17 years old with BMI ≥95th percentile who received a well child visit and assessment for a clinic-based behavioral counseling for obesity. As reported previously, 45.5% were female, 24.9% were white, 11.3% black, 43.5% Hispanic and 12.0% Asian. BMI was expressed as the percent of the 95th percentile for each year of age, and obesity severity was classified as Class I (100-119%), Class II (120-139%), and Class III (≥140%) of the 95th percentile. The proportion of children by age in each obesity subclass overall and by sex and race/ethnicity were examined. Trends in the prevalence of Class III obesity were examined using the Cochrane-Armitage test. Results: Among the 7956 children with obesity, the degree of obesity varied by age, beginning at 15% for Class II and 5% for Class III obesity at age 3 and increasing two-fold to 26-29% for Class II and 12-14% for Class III obesity at age 12-17 (Figure 1). When we further examined trends in Class III obesity, there was a significant linear trend by age (p < 0.001). Although this trend of increasing Class III obesity with age was present for both males and females and persisted for all racial/ethnic groups (p < 0.01, Figure 2) except Asians (p=0.74), the increase in proportion of children with Class III obesity varied by race/ethnicity and appeared to be largest among black children, particularly beginning in early adolescence. Conclusions: Among pediatric patients with obesity, the prevalence of severe obesity (Class II and III) increases with age, regardless of sex and race/ethnicity. Compared to the other racial/ethnic subgroups, black children may be at a higher risk of developing Class III obesity during adolescence. These findings underscore the importance of tracking the severity of obesity in children and adolescents using BMI expressed as a percent of the 95th percentile to allow greater clinical stratification of those in and at high risk of entering obesity Class III. Application of this stratification to population-based obesity research may help identify previously undetected trends, associations, and risk factors for developing severe or sustained obesity into adulthood, which could further inform early clinical interventions.