Objective: To assess if a higher dose of exercise training in exercise-based cardiac rehabilitation could affect improvements in aerobic capacity and muscle strength. Design: Assessor-blinded randomized controlled trial with 12-months follow-up. Setting: Aarhus University Hospital, Aarhus, Denmark. Subjects: A total of 164 cardiac patients referred to exercise-based cardiac rehabilitation were recruited. Interventions: Patients were randomized to 1-hour exercise sessions either three times weekly for 12 weeks (36 sessions, high-dose group) or twice weekly for 8 weeks (16 sessions, low-dose group). The same standardized exercise and intensity protocol including aerobic and muscle strength training was used in all participants. Main measures: Primary outcome was changes in VO2peak. Secondary outcomes were changes in maximal workload, muscle strength and power. Measures were obtained at baseline, after termination of the rehabilitation programme and at follow-up after 6 and 12 months. Results: After the end of intervention, statistically significant between-group differences were seen in favour of the high-dose group in all outcomes: VO2peak 2.6 (mL kg−1 min−1) (95% confidence interval (CI): 0.4–4.8), maximal workload 0.3 W kg−1 (95%CI: 0.02–0.5), isometric muscle strength 0.7 N m kg−1 (95%CI: 0.1–1.2) and muscle power 0.3 W kg−1 (95%CI: 0.04–0.6). After 12 months, a significant between-group difference only persisted in VO2peak and maximal workload. Conclusion: A higher dose of exercise training had a small effect on all outcomes at termination of intervention. A long-term effect persisted in VO2peak and maximal workload. Although the effect was small, it is an important finding because VO2peak is the most important predictor of all-cause mortality in cardiac patients.