The objectives of the current study were to assess in children: (i) The variability of CHIT1 plasma activity by weight status based on BMI-for-age z scores; (ii) The association of dup24 and G102S polymorphism with plasma CHIT1 activity in patients with different levels of obesity by analyzing the observed difference in enzymatic activity between genotype subgroups and the effect of the Bussink et al. [34] correction method; (iii) The contribution of dup24, G102S polymorphism and BMI-for-age z score to the variation of CHIT1 plasma activity. The gene discussed is CHIT1; the disease is obesity due to melanocortin 4 receptor deficiency.