OBJECTIVE: To compare the effects of an 8-week, high-intensity interval training protocol versus continuous training. DESIGN: Randomized controlled trial. SETTING: Cardiac rehabilitation center. PARTICIPANTS: Patients (N=26; mean age ± SD, 54±12y) with chronic heart failure were enrolled in a cardiac rehabilitation program for 8 weeks. INTERVENTIONS: Patients were randomly assigned into 2 groups that performed either interval training (IT) or continuous training (CT). IT consisted of 3 sessions of 12 repetitions of 30 seconds of exercise at very high intensity, followed by 60 seconds of complete rest. The CT group performed CT exercises, which consisted of 45 minutes of aerobic exercise. MAIN OUTCOME MEASURES: Parameters of gas exchanges: peak oxygen consumption (Vo(2)peak), first ventilator threshold (VT1), distance at six-minute walk test (6MWT), and level of anxiety and depression were measured. RESULTS: The IT group increased significantly their Vo(2)peak, the duration of the exercise test, the oxygen pulse, oxygen consumption at the VT1, and the distance walked during the 6MWT. The CT group only increased the time at the VT1 and the distance performed at the 6MWT. The improvement in the time at the VT1 was significantly higher for the IT group than for the CT group. CONCLUSIONS: This study shows that IT at very high intensity for patients with heart failure appears to be more effective than CT in improving indices of submaximal exercise capacity.