Where investments are irreversible and the future is uncertain, people in two countries can make investment decisions that turn out to be mutually inconsistent. I argue that this intertemporal coordination failure explains international business cycles in a two-currency-area setting with a floating foreign exchange rate. The sequence of events starts with an expansionary domestic monetary shock, which decreases the domestic real interest rate. Facing low transactions costs, people spend the new money relatively early in the foreign exchange market and in the foreign market for loanable funds. Domestic monetary expansion thereby changes the relative prices of domestic and foreign goods and also of goods of earlier and later stages of production. The relative price changes lead to intertemporal and international coordination failures once the monetary expansion ends and relative prices change. Domestic monetary policy thereby causes the comovement across different currency areas we observe of business cycles.