Study design: Prospective, randomized, open-label study with blinded endpoint evaluation. Included patients will be randomly allocated to postconditioning or control. Patients with symptoms of acute myocardial infarction of less than 6 hours duration fulfilling ECG criteria for primary PCI are eligible. PCI follow established routines. In postconditioning patients, additional, short (1 min), intermittent balloon occlusions will be applied after initial opening of infarct related artery. After this intervention, PCI proceeds routinely with stent implantation. In the control group, stent implantation after initial opening proceeds as usual. Primary endpoint is final infarct size, determined by MRI after 4 months. 260 patients will be included. Follow-up is 1 year. Inclusion period: 18 - 24 months.