Background:Infant weight and weight gain are positively associated with later obesity, but whether there is a particular critical time during infancy remains uncertain.Objective:The aim was to investigate when and how weight and weight gain during infancy become associated with childhood obesity.Methods:In a cohort representing 28 340 children born from 1959-67 and measured in Copenhagen schools, 962 obese children (2007 World Health Organization criteria), were compared with a 5% randomly selected sub-cohort of 1417 children. Information on weight at birth, 2 weeks, 1, 2, 3, 4, 6 and 9 months was retrieved from health visitors' records. Odds ratios and 95% confidence intervals (CI) for childhood obesity by tertiles of weight at each age and by change in tertiles of weight between two consecutive measurements were estimated using multivariate logistic regression with adjustment for indicators of socioeconomic status, preterm birth, and breastfeeding.Results:Compared with children in the middle weight-tertile, children with a weight in the upper tertile had a 1.36-fold (CI, 1.10-1.69) to 1.72-fold (CI, 1.36-2.18) higher risk of childhood obesity from birth through 9 months, whereas children in the lower weight-tertile had almost half the risk of obesity from 2 through 9 months. The risk of childhood obesity associated with change in weight-tertile in each interval was stable at ∼1.5-fold per weight-tertile increase throughout infancy.Conclusions:Infant weight and weight gain are associated with obesity in childhood already during the first months of life. Determinants of weight gain shortly after birth may be a suitable target for prevention of obesity.