This paper estimates the relationships between market structure and the Lerner index of monopoly constructed from price data on processed food products sold through grocery stores. A theoretical model of a differentiated oligopoly specifies two determinants of price-cost margins: the Herfindahl-Hirschman index of seller concentration adjusted for the elasticity of demand and the industry advertising-to-sales ratio. The results indicate that the three principal determinants of price-cost margin variation, in order of their impacts, are: advertising intensity, elasticity of demand, and concentration. Previous structure-performance studies that did not incorporate the elasticity of demand were probably misspecified.