We propose a concept of intraday overreaction characterized by in-traday price movements which are corrected within the same trading day. It is a concept of relative overreaction in the sense that the ex-tremal intraday price fluctuation is compared with the open-close price change. As a one-sided concept it allows to distinguish between up-ward and downward overreaction. A test for overreaction is proposed and applied to daily high, low, and open-close returns of the compo-nents of the S&P500 and to the German XETRA-DAX stock shares, providing strong support for intraday overreactions to bad news.