To evaluate the resistin/uric acid index as a prognostic factor associated with body mass index (BMI) z-score change after 1 year of lifestyle interventions for obesity. In this prospective cohort study, we included 102 adolescents with overweight or obesity (BMI ≥85th percentile). Weight and height were measured at the start of the lifestyle change intervention and 12 months later. Serum levels of resistin and uric acid were quantified at the beginning of the intervention. The intervention consisted of nutrition education, exercise, and physical activity promotion. The sample included 54 girls and 48 boys; the median age was 11 years (range 10-16 years). The BMI z-score decreased during follow-up (median BMI z-score at baseline was 1.81 vs 1.70 after 1 year, P < .001). The resistin/uric acid index was positively correlated with BMI z-score change (r = 0.27, P < .01). In the linear regression analysis, the resistin/uric acid index was significantly associated with BMI z-score modification at the 12-month follow-up (β = 0.17; 95% CI 0.08-0.26; P < .01). The resistin/uric acid index can be considered a prognostic factor for identifying adolescents with overweight or obesity with a greater probability of improving their BMI. This index could help establish different interventions for adolescents with overweight and obesity; however, additional studies are needed to confirm the usefulness of this index.