Forsum E, et al. - Using valid estimates of body composition, researchers studied correlations between size and body composition variables in infancy and at 4 years of age. The outcomes were compared with those obtained when the body mass index (BMI) was used to estimate 4-year body fatness. Between 2007 and 2015, 253 full-term healthy Swedish children aged 1 week, 12 weeks and 4 years were studied using air displacement plethysmography, size, fat mass and fat-free mass. At 1 and 12 weeks for weight, height, BMI, fat-free mass and fat-free mass index and for fat mass, per cent body fat and fat mass index at 12 weeks, positive associations were found between variables in infancy and at 4 years. During infancy, the fat mass gained was positively associated with the per cent fat mass, fat mass index and BMI at 4 years of age. Gains in fat-free mass in girls were associated with BMI at 4 years of age during infancy. The outcomes provide information on early-life trajectories of body composition and show limitations of BMI as a proxy for body fatness when associated with variables related to early weight gain relevant to later risk of obesity.