Rapid technetium-99 m methoxyisobutylisonitrile (99 mTc-MIBI) washout has been shown to occur in impaired myocardia. This study is based on the hypothesis that scintigraphy can be applied to calculate the myocardial 99 mTc-MIBI washout rate (WR) to diagnose and evaluate heart failure severity and other left ventricular functional parameters specifically in idiopathic dilated cardiomyopathy (IDCM) patients. Patients with IDCMP (n = 17; 52.65 ± 11.47 years) and normal subjects (n = 6; 49.67 ± 10.15 years) were intravenously administered 99 mTc-hexakis-2-methoxyisobutylisonitrile (99 mTc-MIBI). Next, early and delayed planar data were acquired (at 3.5-h intervals), and electrocardiogram (ECG)-gated myocardial perfusion single photon emission computed tomography (SPECT) was performed. The 99 mTc-MIBI WR was calculated using early and delayed planar images. Left ventricular functional parameters were also analyzed using quantitative gated SPECT (QGS) data. In target group, myocardial WRs (29.13 ± 6.68%) were significantly higher than those of control subjects (14.17 ± 3.31%; P < 0.001). The 99 mTc-MIBI WR increased with the increasing severity of the NYHA functional class (23.16 ± 1.72% for class I, 30.25 ± 0.95% for class II, 32.60 ± 6.73% for class III, and 37.50 ± 7.77% for class IV; P = 0.02). The WR was positively correlated with the end-diastolic volume (EDV) index (r (2) = 0.216; β = 0.464; P = 0.02 [ml/m(2)], the end-systolic volume (ESV) index (r (2) = 0.234; β = 0.484; P = 0.01 [ml/m(2)]), the summed motion score (SMS) (r (2) = 0.544; β = 0.738; P = 0.00), and the summed thickening score (STS) (r (2) = 0.656; β = 0.810; P = 0.00); it was negatively correlated with the left ventricular ejection fraction (LVEF) (r (2) = 0.679; β = -0.824; P = 0.00). It can be concluded that 99 mTc-MIBI scintigraphy might be a valuable molecular imaging tool for the diagnosis and evaluation of myocardial damage or dysfunction severity.