As of late 2020, the Trump administration had initiated almost one hundred rollbacks of U.S. environmental regulations. A careful assessment of the benefits and costs of rolling back an existing regulation can and should inform such decisions. When assessing the potential rollback of an existing regulation, analysts can often learn from the regulation’s implementation through retrospective analysis as well as from advances in scientific knowledge. We discuss recent actions concerning the Mercury and Air Toxics Standards (MATS) to illustrate the potential lessons from doing so. In the case of MATS, advances in science have shed light on broader exposure pathways and previously unquantified health effects, suggesting that the benefits of reducing mercury emissions may exceed previous estimates. At the same time, changes in the energy sector have altered the mix of fuels used to produce electricity, which impacts both the benefits and costs of the regulation.