We sought to demonstrate the impact of improved peak exercise oxygen consumption (V̇O ) during maximal exercise testing after cardiac rehabilitation (CR) on the incidence of arrhythmias in patients with heart failure (HF). The present study comprised of 220 patients with HF, and peak V̇O was examined at 2 and 5 months after CR. Of the 220 patients, 110 (50%) had a low peak V̇O of < 14 mL/min/kg at 2 months. The peak V̇O improved in 86 of these 110 (78%) patients at 5 months after CR. During a median follow-up of 6 years, the patients with improvement in peak V̇O , compared to those without peak V̇O improvement, had a lower rate of mortality (4% vs. 29%, log-rank, P < 0.001) and HF hospitalization (6 vs. 17%, log-rank, P = 0.044) and a lower incidence of new-onset atrial arrhythmias (9 vs. 27%, log-rank, P = 0.013), with no difference in the incidence of ventricular arrhythmias between groups (1 vs. 4%, log-rank, P = 0.309). The majority of deaths in the patients without an improved peak V̇O were because of cardiovascular events (73%), particularly progressive HF (55%). Early detection and management of atrial arrhythmias may improve outcomes in patients without peak V̇O improvement after CR.