In this paper, we address the problem of patent valuation. With this aim in view, we focus on the feasibility of a patent rating system. This leads us to develop a structural model of patent renewal decisions based on real options that links patent renewals and patent value and to estimate it on micro level data. Results for a sample of European patents show that unobserved heterogeneity is too high to efficiently discriminate among patents and cast some doubt on the possibility to develop a reliable rating system based only on patent metrics.