Recent developments in the literature on market structure have allowed the generation of a few key testable predictions from the theory of strategic behaviour. The seminal model considers one simple but general relationship, that between market structure and market size, focusing on the competitive roles of endogenous sunk costs in the form of advertising and/or research and development (R&D). Evidence presented in this case study, building on earlier econometric work, shows that such endogenous sunk costs do play a crucial role in the formation of market structure in the global pharmaceutical industry.