ABSTRACT Verrecchia (1983, 1990) introduced the proprietary cost hypothesis in which exogenous disclosure costs are a reduced-form interpretation of lost competitive advantage in product markets. We develop a micro-foundation for this disclosure cost in a Cournot game and explicitly derive the cost as a function of market structure. When the market is sufficiently competitive, this model has a reduced-form representation similar to a standard voluntary disclosure game with a partial disclosure equilibrium. Proprietary costs are increasing in the number of competitors, the degree of product substitution, overall uncertainty, and production costs. The analysis also offers new empirical predictions on the interaction between disclosure choice, managerial horizon, and entry. JEL Classifications: D82; L13; L50; M23; M4.