BACKGROUND Whether high-intensity interval training (HIIT) is more efficient than moderate-intensity continuous exercise (MICE) to increase cardiorespiratory fitness in patients with acute coronary syndrome at moderate-to-high cardiovascular risk is controversial. The best approach to guide training intensity remains to be determined. OBJECTIVE We aimed to assess intensities achieved with self-tailored HIIT and MICE according to perceived exertion and to compare the effect on cardiorespiratory fitness in patients early after ST-elevation myocardial infarction (STEMI). METHODS We included 69 males starting cardiac rehabilitation within 4 weeks after STEMI. After a 3-week run-in phase with MICE, 35 patients were randomised to 9 weeks of HIIT (2×HIIT and 1×MICE per week) and 34 patients to MICE (3×MICE). Training workload for MICE was initially set at the patients' first ventilatory threshold (VT). HIIT consisted of 4×4-min intervals with a workload above the second VT in high intervals. Training intensity was adjusted weekly to maintain the perceived exertion (Borg score 13-14 for MICE, ≥15 for HIIT). Session duration was 38min in both groups. Peak oxygen consumption (VO2) was measured by cardiopulmonary exercise testing pre- and post-intervention. RESULTS Both groups improved peak VO2 (ml/kg/min) (HIIT +1.9, P<0.001; MICE +3.2, P<0.001, Cohen's d -0.4), but changes in VO2 were not significantly different between groups (P=0.104). Exercise regimes did not differ between groups in terms of energy expenditure or training time, but perceived exertion was higher with HIIT. CONCLUSIONS Self-tailored HIIT was feasible in patients early after STEMI. It was more strenuous but not superior nor more time-efficient than MICE in improving peak VO2. The trial was registered at ClinicalTrial.gov (NCT02627586).