AbstractWhen there are costs of trade, such as transport or other costs, the pattern of trade may not be well described by the usual measures of comparative advantage, which simply compare a country's costs or autarky prices to those of the world. Instead, a better comparison takes into account the costs of trade. This paper shows first, in an example, how trade patterns can vary with costs of trade. It then provides restatements of the law of comparative advantage, first in a Ricardian model with trade costs, then extending a 1980 result due to Deardorff and to Dixit and Norman to include trade costs explicitly in a general framework. It uses this latter result to derive two correlations relating trade patterns to measures of comparative advantage that take account of both autarky prices and the costs of trade. Finally, the paper examines the solution to a trade model with product differentiation in order to make the potential role of trade costs more explicit, both algebraically and graphically. With product differentiation either by country or by firm, net trade in an industry, both bilaterally and globally, depends on a country's costs of both production and trade relative to an index of those costs for other countries.