The U.S. State Department requires that all U.S. citizens, including those who have dual citizenship, must be in possession of a valid U.S. passport when leaving and entering the U.S. Likewise, Swedish citizens are expected to declare their nationality and to present a valid Swedish passport upon arrival in Sweden and/or the European Union. Citizens of both Sweden and the United States should therefore bring both passports when traveling between the two countries.> Read more at the U.S. State Department website