AbstractBackground:This study aims to assess the joint association of screen time (ST) and physical activity (PA) with anthropometric indices among Iranian children and adolescents.Methods:In this national study, 23,183 school students, aged 6–18 years, were studied. By using a multi-stage cluster sampling method, they were selected from rural and urban areas of 30 provinces of Iran. ST and PA were assessed by self-administered validated questionnaires. Height, weight, hip and waist circumferences (WC) were measured according to standard protocols, and body mass index (BMI) was calculated. ST of <2 h per day was categorized as “low” and ≥2 h per day as “high”. PA levels were obtained by a 7-day recall of sports or activities that made participants sweat or make their legs feel tired, or games that made them breathe heavily. Using this questionnaire, the score of 1–1.9 was categorized as “low” and scores between 2 and 5 as “high” PA. Logistic regression analyses were used to evaluate the joint association of PA and ST categories with odds of anthropometric measures.Results:In both genders, those students categorized as “Low PA & High ST” had the highest levels of BMI z-scores (boys: 0.15±1.12, girls: 0.17±1.08), WC (boys: 69.93±13.89 cm, girls: 67.30±11.26 cm), and hip circumference (boys: 82.41±13.90 cm, girls: 84.05±13.7 cm), as well as the highest prevalence of overweight (boys: 15.32%, girls: 14.04%) (p<0.001 in all comparisons). In linear multivariate model, students with “High PA & High ST” had significant increased levels of z-scores for BMI, WC and hip circumference (p<0.05).Conclusions:The current findings underscore the importance of reducing ST along with increasing PA for prevention and control of excess weight in children and adolescents.