This paper tests whether upstream R&D cooperation leads to downstream collusion. We show that a sufficient condition for identifying collusive behavior is a decline in the market share of RJV-participating firms. Using information from the U.S. National Cooperation Research Act, we estimate a market share equation correcting for the endogeneity of RJV participation and R&D expenditures. We find robust evidence that large networks between direct competitors - created through firms being members in several RJVs at the same time - are conducive to collusive outcomes in the product market which reduce consumer welfare. By contrast, RJVs among non-competitors are efficiency enhancing.