Background— Successful arrhythmia ablation normalizes ejection fraction (EF) in tachycardia-mediated cardiomyopathy, but recurrent heart failure and late sudden death have been reported. The aim of this study was to characterize the left ventricle (LV) of tachycardia-mediated cardiomyopathy patients long after definitive arrhythmia cure. Methods and Results— Thirty-three patients with a history of successfully ablated incessant focal atrial tachycardia 64±36 months prior, and 20 healthy controls were recruited. At ablation, 18 patients had EF<50% (AT-low EF) that recovered within 3 months from 37±12 to 56±4% ( P <0.001), whereas 15 patients had EF>55% (AT-normal EF). No subjects had EF of 50% to 55%. Subjects underwent echocardiography with speckle tracking and contrast-enhanced cardiac MRI with ventricular T ₁ mapping as an index of diffuse fibrosis. Contrast-enhanced cardiac MRI was performed using a clinical 1.5-T scanner and 0.2 mmol/kg gadolinium-diethylene triamine penta-acetic acid for contrast. Subject characteristics were similar across the 3 groups. Compared with AT-normal EF patients and controls, AT-low EF patients had lower EF (60±6 versus 64±4 and 65±4%; P <0.05), greater indexed LV end-diastolic volume (102±34 versus 84±14 and 85±16 mL/m ² ; P <0.05), and greater indexed LV end-systolic volume (41±11 versus 31±7 and 30±8 mL/m ² ; P <0.01) on contrast-enhanced cardiac MRI. Compared with controls, AT-low EF patients had reduced global LV corrected T ₁ time (442±53 versus 529±61; P <0.05) consistent with diffuse fibrosis. Conclusions— Tachycardia-mediated cardiomyopathy patients exhibit differences in LV structure and function including diffuse fibrosis long after arrhythmia cure, indicating that recovery is incomplete.