Aims: To evaluate the clinical importance of the ST peak phenomenon during primary percutaneous coronary intervention (PCI) in patients with ST-segment elevation myocardial infarction (STEMI). Methods and results: Continuous ST monitoring was performed in 942 STEMI patients from arrival until 90 minutes after revascularisation. ST peak was defined as ≥1 mm increase in the ST-segment during PCI compared with the ST elevation before intervention. ST peak was observed in 26.9% of patients. During median follow-up of 4.1 years, 20.7% of patients experienced a major adverse cardiac event (MACE). ST peak was associated with higher rates of mortality (13.4% versus 9.3%; p=0.044), admission for heart failure (10.6% versus 5.2%; p=0.002) and MACE (26.9% versus 18.2%; p=0.002), but not reinfarction (7.1% versus 5.2%; p=0.14). In two different Cox regression analyses, adjusting for predictors of MACE and ST peak including ST resolution and epicardial flow, ST peak remained significantly associated with MACE: adjusted hazard ratio (HR) 1.40 (95% confidence interval [CI] 1.01-1.95) and 1.41 (95% CI: 1.02-1.96). Conclusions: In the largest study hitherto evaluating the ST peak phenomenon during primary PCI, we demonstrated that ST peak is a strong predictor of adverse long-term outcome and provides independent prognostic information beyond that provided by ST resolution and epicardial flow.