Objective - To investigate whether childhood overweight at age 4-5 increases publicly funded health care costs during childhood, and to explore the role of timing and duration of overweight on health costs. Data Sources - The Longitudinal Study of Australian Children (2004-2008) and linked records from Medicare, Australia s public health insurance provider (2004-2009). Study Design - The influence of overweight status on non-hospital Medicare costs incurred by children over a 5-year period was estimated using two-part models and one-part generalized linear models (GLM). All models controlled for demographic, socioeconomic, and parental characteristics. Principal Findings - Being overweight at age 4-5 is associated with significantly higher pharmaceutical and medical care costs. The results imply that for all children aged 4 and 5 in 2004-2005, those who were overweight had a combined 5-year Medicare bill that was AUD 9.8 million higher than that of normal weight children. Results from dynamic analyses show that costs of childhood overweight occur contemporaneously, and the duration of overweight is positively associated with medical costs for children who became overweight after age 5. Conclusions -This study reveals that the financial burden to the public health system of childhood overweight and obesity occurs even during the first 5 years of primary school.