BACKGROUND AND OBJECTIVES: Several observational studies have revealed that children who receive nonparental childcare are at increased risk of obesity. However, this may be due to unmeasured confounding or selection into different types of childcare. It is not well established whether this association reflects a causal effect of childcare attendance on obesity risk. We examined the effect of attending childcare on children's BMI z scores, using nationally representative data of ∼10 700 children followed from age 9 months through kindergarten entry.