Fifty years after the advent of state newborn screening (NBS) programs for a metabolic condition, there is evidence that the decision to mandate universal screening can reduce health disparities. When in-hospital screening for phenylketonuria began in the early 1960s, most hospitals simply added the procedure to the list of routine clinical practices for newborns, such as giving vitamin K. For a variety of reasons, including fear of missed cases, advocates managed to get state governments involved. By the late 1960s, most states required screening of all or almost all newborns.1 Although these advocates and state legislators did not describe their actions as addressing population-level health disparities, they believed that it was unfair for some infants to bear the consequences of late diagnosis of phenylketonuria simply because they were born in a hospital that did not provide the test. By making NBS for phenylketonuria universally available, they reduced the impact of unequal access to a new and effective therapeutic intervention—one cause of health disparities based on income, location, education, and race/ethnicity.2