The ABX Study examined the effects of antibiotic use in the first two years of life on body mass index (BMI) at ages 5 and 10 years and on growth trajectories to age 5. The observational study used electronic health record (EHR) data for over 362,000 patients from 35 participating healthcare organizations nationwide. Data were collected through the PCORnet distributed research infrastructure from several clinical research networks (CRNs), including REACHnet. Results and publications are available on the study page on PCORI’s website.