Steep APD restitution slopes (> 1) and spatial APD restitution heterogeneity provide the substrate for VF in computational models and experimental studies. Their relationship to ventricular arrhythmia vulnerability in human cardiomyopathy has not been defined. Patients with cardiomyopathy (LVEF < 40%) and no history of ventricular arrhythmias underwent risk stratification with programmed electrical stimulation or T wave alternans (TWA). Low risk patients (n=10) had no inducible VT or negative TWA while high risk patients (n=8) had inducible VT or positive TWA. Activation Recovery Interval (ARI) restitution slopes were measured simultaneously from ten RV endocardial sites during an S1-S2 pacing protocol. ARI restitution slope heterogeneity was defined as the coefficient of variation of slopes. Mean ARI restitution slope was significantly steeper in the high risk group compared with the low risk group (1.16 ± 0.31 vs. 0.59 ± 0.19, p=0.0002). The proportion of endocardial recording sites with a slope > 1 was significantly larger in the high risk patients (47 ± 35 vs. 13 ± 21%, p=0.022). Spatial heterogeneity of ARI restitution slopes was similar between the two groups (29 ± 16 vs. 39 ± 34%, p=0.48). There was an inverse linear relationship between the ARI restitution slope and the minimum diastolic interval (p<0.001). In cardiomyopathic patients at high risk of ventricular arrhythmias, ARI restitution slopes along the RV endocardium are steeper, but restitution slope heterogeneity is similar compared to those at low risk. Steeper ARI restitution slopes may increase the propensity for ventricular arrhythmias in patients with impaired LV function.