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How important is access to patent documents for subsequent innovation? We examine the expansion of the USPTO Patent Library system after 1975. Patent libraries provided access to patents before the Internet. We find that after patent library opening, local patenting increases by 8–20 percent relative to similar regions. Additional analyses suggest that disclosure of technical information drives this effect: inventors increasingly take up ideas from outside their region, and the effect is strongest in technologies where patents are more informative. We thus provide evidence that disclosure plays an important role in cumulative innovation. (JEL D83, K11, O31, O34, R11)