Aim of the study: An inverse correlation between excessive body weight and response to cholecalciferol in vitamin D deficiency (VDD) has been reported but no adjusted or conventional dose and treatment period has universally been recommended by the paediatric guidelines for obese children. In this study, we explored the efficacy of vitamin D supplementation in treatment of children with VDD based on their body mass index (BMI) and different levels of initial serum 25(OH)D. Material and methods: In a single-centre, prospective, open label non-randomized trial in 255 subjects, baseline serum 25(OH)D was measured and different doses of oral D3 prescribed accordingly. Serum D 3 was measured at the end of the treatment period. All statistical analyses were conducted using the statistical package SPSS and p values less than 0.05 considered statistically significant. Results: The response rate to vitamin D supplementation was associated with the patients' BMI characteristics. 25(OH)D levels normalized in 97.7% and 92.7% of the non-obese and obese subjects, respectively. In subjects with BMI ≥ 85 th percentile, there was a lower increase in vitamin D levels after treatment than those with a BMI < 85 th percentile. Evaluating the efficacy of the therapeutic dosage of cholecalciferol as per different categories of vitamin D levels, we observed the highest increase in the level of serum D 3 in the severely deficient D 3 category of both obese and non-obese groups. Conclusions: There is an inverse correlation between high Body Mass Index and response to treatment with vitamin D supplementation, suggesting a higher dose of vitamin D for the optimal treatment of vitamin D deficiency in obese children.