Legislation currently under consideration in the U.S. Congress would increase regulatory oversight of certain diagnostic tests, and a new study by researchers at Massachusetts General Hospital (MGH) and colleagues from several other institutions demonstrates that its potential impact will depend on key details in the bill’s final language. This study, published in JCO Oncology Practice, offers the first evidence-based analysis of how new rules proposed for the regulation of laboratory-developed tests (LDTs) could affect health care costs in the United States.