This working paper examines the efficacy of mass comment campaigns— collections of identical and near-duplicate comments sponsored by organizations and submitted by group members and supporters—in administrative rulemaking in the United States. By assembling information about more than 1,000 mass comment campaigns that occurred during Environmental Protection Agency rulemakings between 2012 and 2016, the analysis addresses the manner in which the agency responds to campaigns and the association between campaigns and the substance of rules. The analysis demonstrates that mass comment campaigns elicit a limited degree of procedural, but not substantive, responsiveness. Procedurally, the vast majority of mass comment campaigns receive responses from the agency. Substantively, however, there is little consistency between mass comment campaigns and the content of rules. These findings suggest that mass comment campaigns have neither enhanced participatory democracy nor transformed rulemaking into a process of “notice and spam."