There is a syndrome consisting of acute infarction-like symptoms and ECG findings, and transient left ventricular apical ballooning without epicardial coronary artery obstruction. A 67-year-old female admitted to our hospital because of severe anterior chest pain was diagnosed as having this syndrome. Since stenotic, spastic, or occlusive sites were not found in epicardial coronary arteries by emergency cardiac catheterization, we speculated coronary microvasculature involvement in the pathophysiology of the event. Four weeks later in a drug-free condition, there was no significant epicardial coronary vasospasm by intracoronary acetylcholine administration (IC-ACh). The average peak flow velocity (APFV) of the left coronary artery (LCA) was measured using the Doppler flow wire method. Under maximal dilatation of the epicardial LCA by intracoronary nitroglycerin administration, IC-ACh was again performed taking into consideration that the change in APFV in response to IC-ACh reflects a coronary microvascular response to it. In the nonischemic control subjects, basal APFV increased to 296+/-29% (n = 24) of the basal value after IC-ACh. In this patient, although IC-ACh did not cause vasospasm in epicardial LCA, APFV was decreased to 54% of its basal value. After administration of a Ca antagonist and KATP opener, she had no chest symptoms and was discharged from the hospital. In 2003, she forgot to take her medication for 3 days and then experienced a sudden recurrence of the same type of attack. She started her medication again and her symptoms disappeared. Three weeks later, she underwent an assessment of the coronary microvascular response to ACh with medicine. Her APFV after ACh increased to 177% of the basal value.