We provide an empirical assessment of the comparative advantage gains from trade argument. We use Japan’s nineteenth-century opening up to world commerce as a natural experiment to answer the following counterfactual: “By how much would real income have had to increase in Japan during its final autarky years of 1851–1853 to afford the consumption bundle the economy could have obtained if it were engaged in international trade during that period?” Using detailed historical data on trade flows, autarky prices, and Japan’s real GDP, we obtain upper bounds on the gains from trade of about 8 to 9 percent of Japan’s GDP.

We exploit Japan’s sudden and complete opening up to international trade in the 1860s to test the empirical validity of one of the oldest and most fundamental propositions in economics: the theory of comparative advantage. Historical evidence supports the assertion that the characteristics of the Japanese economy at the time were compatible with the key assumptions of the neoclassical trade model. Using detailed product‐specific data on autarky prices and trade flows, we find that the autarky price value of Japan’s trade is negative for each year of the period 1868–75. This confirms the prediction of the theory.