The RePEc Economics library offers the largest distributed source of freely downloadable scientific research reports in the world. WoPEc is a user services of that library. It operates on the Internet since 1993. It has a well-established user community, and a relatively narrow subject coverage. In this paper, we wish to find out which papers in the collection are similar through usage. The idea is that if different users request a couple of papers consistently together, then these papers are likely to correspond to the same information needs. They are similar in this sense. We present a theoretical discussion of these relationships and an empirical assessment. We introduce a measure of co-usage and estimate results for the WoPEc user service.