BACKGROUND Hyperlactatemia is a common phenomenon following cardiac surgeries and is associated with prolonged ICU stay and higher morbidity and mortality rates, but such analyses have never focused on patients undergoing heart transplantation (HTX), in whom hyperlactatemia defined with the traditional threshold is observed in nearly every individual. The present study aimed to assess the prognostic value and clinical usefulness of postoperative serum lactate level measurements for in-hospital mortality prediction following HTX. MATERIAL AND METHODS Forty-six consecutive patients who underwent HTX in the Department of Cardiovascular Surgery and Transplantology between 2010 and 2015 were enrolled into a retrospective analysis. Serum lactate level measurements within the first 48 h after HTX were obtained from arterial blood gas analyses, that were routinely conducted every 6 h. Lactate clearance was determined for each patient individually throughout 3 different time frames: the first 24-h (Lac clear 0-24) and second 24-h period (Lac clear 24-48), and the first 48 h after surgery (Lac clear 0-48). RESULTS The ICU admission serum lactate levels differed between the deceased and survivors (7.6 vs. 4.3 mmol/L; p=0.000). Among all tested postoperative lactate level measurements, only the measurement taken upon ICU admission predicted in-hospital mortality (OR 1.94 95% CI [1.09-3.43]; p=0.024). The receiving operating characteristic (ROC) curve for in-hospital mortality was constructed for ICU admission measurement, with the optimal cut-off point estimated at 7.0 mmol/L. CONCLUSIONS Serum lactate level measurement upon ICU admission can be used as a predictive parameter for in-hospital mortality among heart transplant recipients. Values greater than 7.0 mmol/L can predict in-hospital mortality with 90% accuracy.