This paper investigates the effect of the threat and occurrence of patent litigation on the private value of patent protection. Potential challenges are introduced into a renewal model as a factor in patentee decisions as to whether a patent is worth maintaining. The model yields testable predictions about renewal probabilities. Data for post WWII German patents support the hypothesis that the need to defend patent rights influences patentee behaviour. The paper concludes with a discussion of how this factor may be incorporated in patent value estimations and what the results imply for the interpretation of patent data.