Firms have been increasingly relying on online user communities (OUC) to access external, distant knowledge and expertise. Previous research on OUC has largely investigated the influence of individuals' social capital on their knowledge sharing behavior. In this study, we propose a spiral view on the relationship between social capital and knowledge contribution. We suggest that there are two-way interactions between individuals' social capital and their knowledge contribution in online user communities. To test our proposition, we collected and analyzed participation data of 3,512 users from the OUC of BMC, a global leader in innovative software solutions. We discuss the theoretical and practical implications of our study for the online user community literature as well as the broader context of online community.