Prior research on corporate innovation highlights the importance of accessing external knowledge from other firms and universities. However, survey evidence indicates that product users are perhaps the most important source of external knowledge. We build on existing theory to identify the conditions under which user knowledge contributes to corporate innovation and when the benefits will be greatest. Using a panel dataset of medical device companies and their collaborative efforts with innovative physicians, we find evidence that inventive collaborations with users enhance corporate product innovation, and that the benefits are greatest in new technology areas and in the generation of radical innovations.