BACKGROUND AND OBJECTIVES: Obesity persists from childhood to adolescence and into adulthood and is a leading cause of health problems. A positive association between birth weight and body mass index (BMI) among children and adolescents has been shown in many populations. Several studies have indicated that breastfed children have a lower risk of childhood obesity than those who did not. Therefore, this study was to investigate the weight growth trajectory and protective effect of breastfeeding for obesity in children. METHODS: A retrospective cohort study was conducted between January 1, 2008 and December 31, 2016 utilizing data from the National Health Information Database (NHID) of Korea. The total number of 38,039 participants was followed until the end of 2016, provided that participants were completely eligible for all health check-ups from birth to 6 years of age. At each check-up period, multiple logistic regression was used to investigate the association between three birth weight status and growth development categorized into three groups, overweight/obese, normal and underweight. Infants were categorized into three groups by birth weight; low birth weight (LBW) defined as birth weight less than or equal to 2500g,24 normal birth weight (NBW) defined as birth weight greater than 2500g and less than 4000g, and high birth weight (HBW) defined as birth weight greater than or equal to 4000g. Underweight group was defined if percentage of BMI was less than and equal to 10%, overweight/obesity group if the percentage of BMI was greater than and equal to 85%, and others were allocated to the normal weight group. RESULTS: During the follow-up period, about 10% of infants in LBW and 15% of NBW became overweight/obese. However, more than 25% of HBW infants became overweight/obese. HBW infants are highly likely to be overweight/obese compared to NBW infants (OR 1.70~2.35) until 6 years and LBW infants have tendency to be underweight (OR 1.69~2.20) through 6 years. The risk of overweight/ obesity decreases significantly if HBW infant get exclusively breast-feeding for first 6 months (OR 0.54~0.79). CONCLUSIONS: Birth weight status is related to overweight/obesity during early childhood. Exclusive breastfeeding is a significant protective factor against overweight/obese in children with HBW.