An ongoing debate in patent law involves the role that “non-practicing entities,” sometimes called “patent trolls” serve in the patent system. Some argue that they serve as valuable market intermediaries and other argue that they are a drain on innovation and an impediment to a well-functioning patent system. In this article, we add to the data available in this debate by conducting an event study that analyzes the market reaction to patent litigation filed by large, “mass-aggregator” NPE entities against large publicly traded companies. This study advances the literature by attempting to reproduce the results of previous event studies done in this area on newer market data and also by subjecting the event study results to more rigorous statistical analysis. In contrast to a previous event study, in our study we found that the market reacted little, if at all, to the patent litigation filed by large NPEs.