This study investigates the relationships among supply chain members and its impact on the operationalization of manufacturing flexibility. It is proposed a conceptual model and conducted a field work involving four manufacturing companies to study the influence of four aspects of buyer-supplier relationship (trust/commitment, information sharing, supplier development and joint product development) on three external dimensions of manufacturing flexibility (mix, new products and volume). From the field work the buyersupplier relationships are analyzed and their general characteristics and effects on the operationalization of manufacturing flexibility and manufacturing performance are presented. The analysis of scope and achievability factors indicated little explored flexible options, the presence of potential flexibility or the underutilization of flexible resources in the companies. Finding ways for operationalizing flexibility must be prioritized in order to obtain competitive advantage in manufacturing systems. This work shows how buyer-supplier relationships can influence external dimensions of manufacturing flexibility and manufacturing performance.