This paper examines the effects of sunk costs and potential competition on pricing behavior in monopoly airline markets. We find little evidence to support the proposition from contestable markets theory that the level of sunk costs influences pricing by monopolists. Rather, the results support the view, consistent with numerous game theoretic oligopoly models, that the costs and the price-cutting reputation of potential entrants influence incumbent behavior. These results suggest that contestability theory may not be robust. Even in the markets characterized by increasing returns to scale, the perfectly contestable market may not be a useful welfare standard.