We estimate the dynamic causal effects of changes in market power using an instrumental variables approach. We build an instrument for changes in market power on the basis of the asset price implications of antitrust activity. We produce a narrative account of Department of Justice and Federal Trade Commission case filings for antitrust activities for a sample that starts in 1965. We then use criminal and civil non-merger case filings to estimate the impact on excess equity returns of firms – and their competitors – listed on the SEC that are investigated for antitrust. We find that antitrust investigations trigger negative excess returns for firms that are investigated and positive excess returns for their competitors. We convert these excess returns into dollar values and use them as instruments for changes in market power and find that lower market power stimulates aggregate output, consumption, and investment and is associated with lower inflation.