This paper examines the stock market reaction to research and development (R and D) announcements made by listed UK companies. R and D projects on average are found to be associated with significant positive abnormal returns. However, the level of these abnormal returns varies significantly with the ownership structure of the firm. In particular, it is found that the level of abnormal returns are significantly lower for companies with large institutional investors. This negative relationship may be associated with short-term pressures on the performance of institutional investors.