A retrospective review of 50,0261 neonates across 127 neonatal intensive care units (NICUs) from California showed a forty-fold variation in NICU antibiotic prescribing practice with similar burdens of proven infection and mortality. A large number of preterm infants are thus subjected to a potentially harmful course of antibiotics that provides no clear benefit. There remains a major gap in our understanding of antibiotic-related intestinal microbial dysbiosis and how this may result in disease.