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This paper proposes a new channel through which international trade affects macroeconomic volatility. We study a multi-country model with heterogeneous firms that are subject to id-iosyncratic firm-specific shocks. When the distribution of firm sizes follows a power law with exponent sufficiently close to −1, the idiosyncratic shocks to large firms have an impact on aggregate volatility. Opening to trade increases the importance of large firms to the economy, thus raising macroeconomic volatility. We next explore the quantitative properties of the model calibrated to data for the 50 largest economies in the world. Our simulation exercise shows that the contribution of trade to aggregate fluctuations depends strongly on country size: in an economy such as the U.S., that accounts for one-third of world GDP, international trade increases volatility by about 3.5%. By contrast, trade increases aggregate volatility by some 30 % in a small open economy, such as Belgium or Poland. The model performs well in matching the elasticity of macroeconomic volatility with respect to country size observed in cross-countr