Previously, we found that excess weight already in childhood has positive associations with endometrial cancer, however, associations with changes in body mass index (BMI) during childhood are not well understood. Therefore, we examined whether growth in childhood BMI is associated with endometrial cancer and its sub-types. A cohort of 155,505 girls from the Copenhagen School Health Records Register with measured weights and heights at the ages of 6 to 14 years and born 1930-89 formed the analytical population. BMI was transformed to age-specific z-scores. Using linear spline multilevel models, each girl's BMI growth trajectory was estimated as the deviance from the average trajectory for three different growth periods (6.25-7.99, 8.0-10.99, 11.0-14.0 years). Via a link to health registers, 1020 endometrial cancer cases were identified, and Cox regressions were performed. A greater gain in BMI during childhood was positively associated with endometrial cancer but no differences between the different growth periods were detected in models adjusted for baseline BMI. The hazard ratios for the associations with overall growth during childhood per 0.1 z-score increase were 1.15 (95% confidence interval [CI]: 1.07-1.24) for all endometrial cancers, 1.12 (95% CI: 1.04-1.21) for estrogen-dependent cancers, 1.16 (95% CI: 1.06-1.26) for endometrioid adenocarcinomas and 1.46 (95% CI: 1.16-1.84) for non-estrogen-dependent cancers. Growth in BMI in early life is positively linked to later endometrial cancer risk. We did not identify any sensitive childhood growth period, which suggests that excess gain in BMI during the entire childhood period should be avoided.