We report a 30-year-old woman who developed an acute myocardial infarction at 24 weeks of gestation. She did not undergo any kind of acute intervention. On the 8th hospital day, premature delivery was performed safely following premature rupture of membrane. Coronary angiogram was normal and no spasm was induced by provocative test with ergometrine maleate. The patient had abnormal values of fibrinogen, thrombin-antithrombin III complex and plasmin-alpha 2-plasmin inhibitor complex. Thus, thrombus formation might have been associated with the onset of acute myocardial infarction. This is the first case of acute myocardial infarction during pregnancy, showing normal coronary angiogram and negative pharmacological provocation of coronary spasm.