In patients admitted for heart failure (HF) with reduced left ventricular ejection fraction (LVEF) and a concomitant supraventricular tachyarrhythmia (SVT) it is a challenge to predict LVEF recovery and differentiate tachycardia-induced cardiomyopathy (TIC) from dilated cardiomyopathy (DCM). The role of the electrocardiogram (ECG) and cardiac magnetic resonance (CMR) and in this acute setting remains unsettled. Forty-three consecutive patients admitted for HF due to SVT and LVEF < 50% undergoing CMR in the acute phase, were retrospectively included. Those who had LVEF > 50% at follow up were classified as TIC and those with LVEF < 50% were classified as DCM. Clinical, CMR and ECG findings were analyzed to predict LVEF recovery. Twenty-five (58%) patients were classified as TIC. Patients with DCM had wider QRS (121.2 ± 26 vs 97.7 ± 17.35 ms; p = 0.003). On CRM the TIC group presented with higher LVEF (33.4 ± 11 vs 26.9 ± 6.4%; p = 0.019) whereas late gadolinium enhancement (LGE) was more frequent in DCM group (61 vs 16%; p = 0.004). On multivariate analysis, QRS duration ≥ 100 ms (p = 0.027), LVEF < 40% on CMR (p = 0.047) and presence of LGE (p = 0.03) were independent predictors of lack of LVEF recovery. Furthermore, during follow-up (median 60 months) DCM patients were admitted more frequently for HF (44 vs 0%; p < 0.001) than TIC patients. In patients with reduced LVEF admitted for HF due to SVT, QRS ≥ 100 ms, LVEF < 40% and LGE are independently associated with lack of LVEF recovery and worse clinical outcome.