Idiopathic ventricular arrhythmias (IVAs) refers to ventricular tachycardias or premature ventricular contractions (PVCs) in structurally normal hearts without myocardial scar. Strain by speckle tracking echocardiography (STE) can detect subtle changes in myocardial function; however, STE in the evaluation of IVA patients has not been performed. We evaluated STE strain in IVA patients (at baseline, and following radiofrequency ablation (RFA)), to identify IVA-related abnormalities in myocardial substrate. Using STE, global longitudinal strain (GLS), mechanical dispersion (MD), and delta contraction duration (DCD) were measured in 50 consecutive IVA patients undergoing RFA, and compared to 50 healthy controls. IVA ablation success was determined by multi-day Holter monitoring at follow-up. We additionally evaluated 10 IVA patients who declined ablation and 10 patients with PVC-induced cardiomyopathy as comparators. IVA patients at baseline had impaired GLS (-18.7±2.3% versus -21.7±1.6%, p<0.001), increased MD (47.4±10.5ms versus 32.9±6.8ms), and prolonged DCD (133.0±19.0ms versus 71.0±10.0ms) compared to controls. At follow-up (14±3 months), 40 patients (80%) remained free of IVAs with normalisation of GLS. However, MD and DCD remained prolonged compared to controls (40.3±4.1ms versus 32.3±4.2ms and 105±12ms versus 72±10ms respectively). In patients without cure (n=10, 20%), GLS, MD, and DCD significantly worsened at follow-up and was similar to patients who declined RFA and patients with PVC-induced cardiomyopathy. Patients with IVAs manifest subclinical LV dysfunction and impaired myocardial mechanics. These changes are only partially reversed in patients successfully treated with RFA, indicating a likely underlying myocardial substrate abnormality with the need for ongoing active patient surveillance.