The bargaining theory of capital structure implies that when¯rms raise their leverage, their suppliers will raise their own leverage in response, so as to maintain strength in negotiations with important customers. In contrast, the theory of¯rm-speci¯c investments implies that when a customer raises its leverage, a¯rm will respond by lowering its own leverage to minimize the risk of bankruptcy. We test these theories by examining the relationship between the leverage decisions of suppliers and customers. We¯nd that a¯rm's leverage is positively associated with its customer's leverage. Moreover, consistent with the bargaining theory, we¯nd that the positive leverage relationship is stronger if the customer has a higher ex-ante bargaining power. We also¯nd some support for the relation-speci¯c investment theory of capital structure in that the positive leverage relationship is weaker if the supplier-customer relationship requires more relation-speci¯c investments.