Myocardial edema and inflammation play an important role in dilated cardiomyopathy (DCM). This pathologic condition can be identified noninvasively using cardiovascular magnetic resonance imaging (CMR). The purpose of this study was to determine the effectiveness of T2 values obtained with T2 mapping in the detection of edema in DCM patients, compared with that of conventional T2-weighted imaging (T2WI). CMR was used for 15 normal controls (NML) and 26 DCM patients. The DCM patients were classified as having either mild dysfunction with a left ventricular ejection fraction (EF) >35% or severe dysfunction with an EF ≤35%. Myocardial edema was assessed by both T2 mapping and T2WI. The differences between the T2 values determined from T2 mapping and the T2 ratios that were calculated from the T2WI were compared among the NML, mild DCM, and severe DCM patients. The T2 values for the NML, mild DCM, and severe DCM patients were 51.2 ± 1.6, 61.2 ± 0.37, and 67.4 ± 6.8, respectively (P < 0.05 for each pair), and the corresponding T2 ratios were 1.88 ± 0.09, 2.12 ± 0.37, and 2.04 ± 0.34, respectively (P > 0.05). T2 mapping clearly showed that the myocardial water content was larger in DCM patients than in NML controls and that the myocardial water content increased as the disease progressed. Thus, T2 mapping is a useful technique for the diagnosis and quantitation of diffuse myocardial edema.