Antibiotics are among the most valuable medical discoveries. Recent research on understanding of how bacteria in our gut use energy, however, raises concerns about whether broad-spectrum antibiotics, which are over-prescribed for mild infections, prescribed in early infancy may cause obesity during childhood. Obesity is a very common and serious condition among US children, particularly children from disadvantaged populations. Past studies examining the link between antibiotic use and childhood obesity are too small and they lack diversity as well as modern scientific tools to gauge the extent to which prescribing antibiotics can lead to excess weight gain. PCORnet, the National Patient-Centered Clinical Research Network, comprising very large networks of data from electronic medical records, provides an ideal test bed to address this question. Further, it is not clear how caregivers and clinicians will use the results of earlier studies to decide which antibiotics to prescribe when faced with common infections like ear infections.