We estimate the effect of new private-sector unionization on publicly traded firms’ equity value in the United States over the 1961–1999 period using a newly assembledsample of National Labor Relations Board(NLRB) representation elections matchedtostockmarket data. Event-studyestimates showanaverageunion effect on the equity value of the firm equivalent to $40,500 per unionized worker, an effect that takes 15 to 18 months after unionization to fully materialize, and one that could not be detected by a short-run event study. At the same time, point estimates from a regression discontinuity design—comparing the stock market impact of close union election wins to close losses—are considerably smaller and close to zero. We find a negative relationship between the cumulative abnormal returns and the vote share in support of the union, allowing us to reconcile these seemingly contradictory findings. JEL Codes: J01, J08, J5, J51.