Blood lactate levels during cardiopulmonary bypass are often used to verify adequacy of perfusion. The present investigation aimed to propose a threshold for hyperlactatemia. Blood lactate levels in 5 121 cardiac surgical patients were retrospectively analysed by a review of database records. Hyperlactatemia was defined as a value equal to the 90th percentile of the identified lactate distribution at weaning from cardiopulmonary bypass. Patient demographics, background and outcome statistics were performed stratified on presence of hyperlactatemia. The threshold for hyperlactatemia was found to equal 2 mmol/l. Significant predictors of hyperlactatemia based on logistic regression modelling were age, complex surgery, duration of cardiopulmonary bypass, blood transfusion, acid base level, emergency operations, diabetes, vasoactive intervention, venous-blood-return to the heart-lung machine and renal function. Patients with hyperlactatemia required longer intensive care and postoperative ventilatory support. Complications were more frequent, especially: renal dysfunction, infections, respiratory and circulatory disorders. Hospital mortality was 13.3% compared to an overall level at 2.2%. The threshold for hyperlactatemia during cardiopulmonary bypass attained 2 mmol/l and predicted increased morbidity and mortality.