There is a growing literature that aims at assessing the private value of knowledge assets and patents. It has been shown that patents and their quality as measured by citations received by future patents contribute significantly to the market value of firms beyond their R&D stocks. This paper goes one step further and distinguishes between different types of forward citations patents can receive at the European Patent Office. While a patent can be cited as non-infringing state of the art, it can also be cited because it threatens the novelty of patent applications ('blocking citations'). Empirical results from a market value model for a sample of large, R&D-intensive U.S., European and Japanese firms show that patents frequently cited as blocking references have a higher economic value for their owners than patents cited for nonblocking reasons. This finding adds to the patent value literature by showing that different types of patent citations carry different information on the economic value of patents. The result further suggests that the total number of forward citations can be an imprecise measure of patent value.