Purpose: The main objective of this study is to research whether an announcement of an acquisition generates positive/negative abnormal short-term return towards the buying company’s shareholders. The secondary purpose is to research whether any differences could be due to selected factors: firm size and industry. Method: The study is quantitative in nature where the research aims at the stock price movement around the announcement of an acquisition. The sample size includes 30 companies between the years 2000-2010. The abnormal return is investigated by an Event Study. Conclusion: Our study shows that the publication gives a positive abnormal return in comparison to the respective sector indexes. The result is statistically significant, so we can conclude that an announcement of an acquisition is a positive investment on average. We also found that our selected factors have a significant impact on price performance. Medium-sized industrial companies show positive abnormal returns while smaller IT companies shows a very low abnormal return and large finance companies fluctuates slightly more than other variables, but the value changes are minimal. We also found that the market does not work effectively because of the strange movements before and after the event date.