BACKGROUND: One of the challenging risk factors for severe COVID-19 infection is obesity and high body mass index (BMI). In this study we assessed the association between BMI and outcomes of hospitalized pediatric patients with COVID-19 in Iran. METHOD: This retrospective cross-sectional study was performed in the biggest referral pediatric hospital in Tehran from March 7 to August 17, 2020. All hospitalized children ≺18 years of age with laboratory-confirmed COVID-19 were included in the study. We evaluated the association of BMI with COVID-19 outcomes (death, severity of clinical course, oxygen therapy, intensive care unit [ICU] admission, ventilator requirement). The secondary objectives were investigating the association of gender, underlying comorbidity, and patient age with COVID-19 outcomes. The limits for obesity, overweight, and underweight were set at BMI >95 percentile, 85≤ BMI ≤ 95, and BMI<5 percentile, respectively. RESULTS: In total, 189 confirmed pediatric cases of COVID-19 (0.1-17 years) with a mean age of 6.4 ± 4.7 years were included. Overall, 18.5% of the patients were obese and 33% were underweight. We found that BMI had no significant relation with COVID outcomes in pediatric cases but after subgrouping the participants, underlying comorbidities and lower BMI in previously ill children were independently associated with a poor clinical outcome of COVID-19. In addition, the previously ill children with higher BMI percentiles were at a relatively lower risk of ICU admission (95% CI: 0.971-0.998, OR: 0.98, p = 0.025) and better clinical course of COVID-19 (95% CI: 0.970-0.996, OR: 0.98, p = 0.009). The BMI percentile had a statistically significant direct relationship with age (Spearman correlation coefficient= 0.26, p<0.001). When we separated the children with underlying comorbidity, the BMI percentile was significantly lower (p<0.001) in comparison to the previously healthy children. CONCLUSION: Based on our results, obesity is not related to COVID-19 outcomes in pediatric patients, but after controlling for confounding effects, underweight in children with underlying comorbidities was more likely to be associated with a poor prognosis of COVID-19.