How do firms' sales interact across markets? Are foreign and domestic sales complements or substitutes? Using a French firm-level database combining balance-sheet and product-destination-specific export information over the period 1995–2001, we study how demand conditions in foreign markets affect domestic sales through variations in exports. We identify a number of exogenous shocks affecting the firms' demand on foreign markets, including product-destination specific imports or tariff changes, and large foreign shocks such as financial crises or civil wars. Our results show that exogenous variations in firm-level exports positively impact domestic sales, even after controlling for domestic demand conditions. A 10% exogenous increase in foreign sales generates a 1 to 3% increase in domestic sales in the short-run. This result is robust to various estimation techniques, instruments, controls, and sub-samples. It is also supported by the natural experiment of the Asian crisis in the late 1990's.