Most traditional research on mergers and acquisitions tends to focus on the role of similarity in explaining acquisition performance. While scholars have recently begun to examine acquisition complementarity, there is still little evidence concerning how complementarity influences acquisition performance. Further, previous research has not drawn the connections between related contexts and the potential benefits from complementarity. In this article, we move the study of acquisition complementarity forward by investigating the effects of strategic and market complementarity on acquisition performance in the context of related horizontal acquisitions. We also propose that two key attributes of acquirers—strategic focus and out-of-market acquisition experience—will moderate this relationship. We investigate our research questions in the context of all 2,204 acquisitions made by publicly traded U.S. commercial banks during the 12-year period from 1989 to 2001. Our findings are generally supportive, suggesting complementarity is an important antecedent of acquisition performance, and raising important issues on the nature of acquisition research in general. Copyright © 2009 John Wiley & Sons, Ltd.