We document that the first leak of customer information from a tax-haven bank caused a sudden flight of deposits from tax havens and a sharp decrease in the market value of banks known to be assisting with tax evasion. The loss of market value was largest for the banks most strongly involved in tax evasion. Subsequent leaks had qualitatively similar although smaller effects. Our findings suggest that whistleblowing in tax-haven banks deters offshore tax evaders by increasing the perceived risk of committing and assisting with tax evasion.