Aims To evaluate safety and efficacy of moderate intensity interval exercise training early after heart failure decompensation on exercise tolerance and health-related quality of life (HRQoL). Methods and results This is a prospective randomized controlled study. We screened 234 consecutive patients admitted with decompensated heart failure; 46 patients (42 men/4 women; 61 ± 12 years of age) were randomized to a moderate intensity aerobic interval training (n = 24) or to a control group (n = 22). Patients underwent cardiopulmonary exercise testing, echocardiography and Minnesota Living with Heart Failure questionnaire (MLHFQ) at baseline, after three weeks and after three months. After three weeks, peak-VO2 increased by 17% in the training group (p = 0.003) with further increase by 10% after three months (p < 0.001) but did not change significantly in controls. MLHFQ score improved after three weeks, with better results in the training group (from 64.6 ± 15.6 to 30.8 ± 12.9, p < 0.001). After three months, MLHFQ further improved in the exercise training group, but not in controls. Left ventricular ejection fraction was not significantly different between the two groups at baseline and after three months. No serious adverse events related to exercise testing or training were observed. Conclusions Interval exercise training early after an episode of heart failure decompensation is safe and effective in improving exercise tolerance and health-related quality of life in selected patients after achievement of clinical stability. Positive effects remained sustained after three months. Further studies are needed to define role and indications for interval exercise training early after heart failure decompensation.