The Trade-Related Aspects of Intellectual Property Rights Agreement, administered by the World Trade Organization, ensures the smooth functioning of the international patent system. It promises among other things that local and foreign firms are treated in the same, nondiscriminatory manner. We test for whether the national treatment principle has been upheld in the five largest patent offices and document the existence of a systematic bias against foreign firms in patent examination decisions. We find that filing international patent applications under the Patent Cooperation Treaty can reduce some of the bias.