Objectives:Studies regarding the association of child-care use with body mass index (BMI), overweight or obesity development show contradictory results. This study examined the relationship between child-care use and BMI z-scores and overweight, as well as associates of child-care use in children up to 2 years old.Methods:Longitudinal data originated from the Dutch KOALA Birth Cohort Study. Questionnaires assessed child-care use at ages 7 months and 1 and 2 years (N=2396). Height and weight assessed at 1 and 2 years were used to calculate BMI z-scores. Overweight was defined as a BMI z-score of >/=85th percentile. The influence of child-care use on weight development was tested using backward linear and logistic regression analyses. Outcomes were: (1) BMI z-score at 1 and 2 years; (2) change in BMI z-score between 1 and 2 years; (3) overweight vs non-overweight at 1 and 2 years; and (4) change from normal weight to overweight vs remaining normal weight between 1 and 2 years. The association between child-care use and parental background characteristics was tested using backward logistic regression analyses.Results:Child-care use (no/yes) at 1 and 2 years positively predicted BMI z-scores at age 2 years, as well as change in BMI z-score between 1 and 2 years. These associations were adjusted for various covariates (for example, parental working hours). Furthermore, child-care use significantly increased the odds of being overweight at age 1year. There were few differences in BMI or overweight between intensive (>16 h per week) and limited child-care use (</=16 h). Child-care use was positively associated with various parental characteristics, including parental working hours and maternal educational level.Conclusion:The findings suggest a small influence of child-care use on weight development in very young children. The child-care setting could have an important role in preventive interventions against overweight and obesity development in young children.International Journal of Obesity advance online publication, 25 May 2010; doi:10.1038/ijo.2010.100.