We present an empirical analysis of the effects of patent thickets at the European Patent Office on entry into patenting by UK firms. Using a direct measure of patent thicket density, we provide evidence for the existence and growth of patent thickets in specific industries, notably in telecommunications, audiovisual technology, and computer technology. Our analysis indicates that the density of patent thickets is associated with reduced entry into patenting in the particular technology area (controlling for the level of patenting in that area). We find this effect to be particularly pronounced for electronics and telecommunications. It is also stronger for smaller than for large companies.