The purpose of this thesis is to perform event studies that determine the level of efficiency of twenty-two large cap stocks from the OMXS30 Index. Under the event study methodology, analysts’ expectations, the standard Capital Asset Pricing Model and a set of parametric tests are implemented. As a result, significant evidence is found on the existence of intraday abnormal returns at the exact moment of an interim report publication. However, further evidence on market corrections state efficient market behavior.