Rising levels of childhood overweight and obesity represent a major global public health challenge. A number of studies have explored the association between childhood overweight and obesity and healthcare utilisation and costs. This paper adds to the literature by estimating the causal effect of child overweight and obesity status on use of general practitioner (GP) and hospital inpatient stays at two time points using instrumental variable (IV) methods The paper uses data from two waves of the Growing Up in Ireland survey of children when they are 9 and 13 years respectively and uses the biological mother's body mass index (BMI) as an instrument for the child's BMI. Our results demonstrate that child overweight and obesity status do not have a significant effect on healthcare utilisation for children when they are 9 years, but do have a large and significant effect at 13 years. Across all our models, the effects on both GP and hospital inpatient stays are found to be larger when endogeneity in childhood BMI status is addressed. Previous studies that did not address endogeneity concerns are likely to have significantly underestimated the impact of child overweight and obesity status on healthcare utilisation.