In this paper, the authors analyse the stock price effects of block transactions and the disclosure thereof for Dutch firms. The authors measure a significant positive price effect for block purchases directly around the transaction date, but not for block sales. The authors do not measure a significant price effect for the disclosures of block transactions. The authors find two opposing effects about the agency role of large shareholders. In particular, while the authors find that the market reacts positively to the entrance of a new block holder, the authors also measure a negative reaction to the purchase of large blocks.