Exercise intolerance is a symptom of chronic heart failure (CHF). The magnitude of exercise tolerance, as measured by peak oxygen uptake (peak VO), is strongly associated with prognosis in patients with CHF. We aimed to evaluate the factors associated with improved exercise tolerance in patients with HF. In this prospective study, we recruited patients who were diagnosed with non-ischemic cardiomyopathy between September 2017 and September 2021. All patients underwent cardiopulmonary exercise testing at discharge and 6 months after enrollment. The patients were stratified according to whether peak VO was increased or not at 6 months. One hundred patients with a reduced left-ventricular ejection fraction (LVEF < 50%) were enrolled. Improvement of peak VO was observed in 74 patients. In male patients, hemoglobin level was higher in the increased peak VO group than in the non-increased group (15.0 ± 1.9 g/dL vs. 13.1 ± 2.1 g/dL; p < 0.01). Baseline hemoglobin level was positively correlated with the percentage change in peak VO (Spearman's r = 0.248, p = 0.040). Kaplan-Meier analysis demonstrated that adverse cardiac events were significantly less frequent in the increased peak VO group than in the non-increased group (log-rank test, p = 0.032). Multivariate logistic regression analysis identified hemoglobin level as an independent predictor of improved peak VO [odds ratio (OR) 1.60; 95% confidence interval (CI) 1.05-2.44; p = 0.027]. Baseline hemoglobin level is an independent predictor of improved peak VO in male patients with non-ischemic cardiomyopathy.