BackgroundArrhythmogenic cardiomyopathy (AC) is characterized by biventricular dysfunction, exercise intolerance, and high risk of ventricular tachyarrhythmias and sudden death. Predisposing factors for left ventricular (LV) disease manifestation and its prognostic implication in AC are poorly described. We aimed to assess the associations of exercise exposure and genotype with LV dysfunction in AC, and to explore the impact of LV disease progression on adverse arrhythmic outcome.Methods and ResultsWe included 168 patients with AC (50% probands, 45% women, 40±16 years old) with 715 echocardiographic exams (4.1±1.7 exams/patient, follow‐up 7.6 [interquartile range (IQR), 5.4–10.9] years) and complete exercise and genetic data in a longitudinal study. LV function by global longitudinal strain was −18.8% [IQR, −19.2% to −18.3%] at presentation and was worse in patients with greater exercise exposure (global longitudinal strain worsening, 0.09% [IQR, 0.01%–0.17%] per 5 MET‐hours/week,P=0.02). LV function by global longitudinal strain worsened, with 0.08% [IQR, 0.05%–0.12%] per year; (P<0.001), and progression was most evident in patients with desmoplakin genotype (Pfor interaction <0.001). Deterioration of LV function predicted incident ventricular tachyarrhythmia (aborted cardiac arrest, sustained ventricular tachycardia, or implantable cardioverter defibrillator shock) (adjusted odds ratio, 1.1 [IQR, 1.0–1.3] per 1% worsening by global longitudinal strain;P=0.02, adjusted for time and previous arrhythmic events).ConclusionsGreater exercise exposure was associated with worse LV function at first visit of patients with AC but did not significantly affect the rate of LV progression during follow‐up. Progression of LV dysfunction was most pronounced in patients with desmoplakin genotypes. Deterioration of LV function during follow‐up predicted subsequent ventricular tachyarrhythmia and should be considered in risk stratification.