The aim of this study is to evaluate the agreement between myocardial F-FDG PET imaging and cardiac magnetic resonance imaging (cMRI) in assessing cardiac function and relationship of cMRI late gadolinium enhancement (cMRI-LGE) and myocardial perfusion/metabolism pattern in patients with idiopathic dilated cardiomyopathy (IDCM). Forty-two consecutive patients diagnosed with IDCM were enrolled. All patients underwent Tc-MIBI SPECT, gated F-FDG PET imaging, and cMRI within 3-7 days. Cardiac function parameters were calculated using PET and cMRI. The segments analysis was performed using a 17-segment model. Patterns of perfusion/metabolism were classified as normal, mismatch, mild-to-moderate match, and severe match, and cMRI-LGE was classified into 3 categories (non-LGE, mid-wall LGE, and transmural LGE). The correlation between gated PET and cMRI was excellent for end-diastolic volume (EDV; r = 0.948, P < 0.001), end-systolic volume (ESV; r = 0.939, P < 0.001), and left ventricular ejection fraction (LVEF; r = 0.685, P < 0.001). EDV and ESV were underestimated, whereas LVEF was slightly overestimated by gated PET in comparison to cMRI. Perfusion/metabolism patterns varied in 3 different categories of non-LGE, mid-wall LGE, and transmural LGE (χ = 14.276, P < 0.001). Also, 71.0% (44/62) segments with mid-wall LGE had normal perfusion/metabolism patterns, and 75.9% (63/83) perfusion/metabolism mismatch segments were shown as non-LGE. The incidence of LGE was significantly higher in segments with severe match than the other 3 segment groups (χ = 112.53, P < 0.001). There is an excellent agreement between gated PET and cMRI in assessment of cardiac function. LGE-cMRI is much more sensitive in detecting moderate fibrosis, while PET could detect more impaired but viable myocardium. Combining the 2 imaging modalities is useful for providing more comprehensive evaluations of myocardial injury in patients with IDCM.