Use of anatomic testing such as coronary artery calcium scoring (CACS) of 0 to avoid myocardial perfusion imaging in symptomatic patients could lead to missing microvascular dysfunction in 4 out of 10 patients, a finding associated with a high mortality risk. This study aimed to assess the relative prognostic importance of myocardial blood reserve (MBFR), perfusion defects, and CACS in patients with suspected coronary artery disease. Previous intermodality comparisons have been limited by the lack of the simultaneous, same-patient acquisition of PET and SPECT, and use of semiquantitative or visual perfusion assessment.