Patent law both imposes a duty on patent applicants to submit relevant prior art to the PTO and assumes that examiners use this information to determine an application’s patentability. In this paper, we test the validity of these assumptions by studying the use made of applicant-submitted prior art by delving into the actual prosecution process in over a thousand different cases. We find, to our surprise, that patent examiners effectively ignore almost all applicant-submitted art, relying almost exclusively on prior art they find themselves. Our findings have significant implications for a number of important legal and policy disputes, not least of which is the soundness of the strong presumption of validity the law grants issued patents.