In patients suffering from a variety of severe diseases the detection of erythroblasts in peripheral blood is associated with poor prognosis. However, as yet the prognostic significance of erythroblasts in the blood of patients after cardiothoracic surgery has not been assessed. In a retrospective study we analyzed the database of 2074 patients, of whom 87 died in hospital during the postoperative period. All patients underwent cardiothoracic surgery using a heart-lung machine. Together with erythroblasts in blood, age, sex, body mass index, preoperative ejection fraction, smoking, diabetes mellitus, type of operation, emergency surgery, renal deficiency, pulmonary hypertension, and endocarditis were considered. The postoperative mortality of patients with erythroblasts in peripheral blood (n=57) was 45.6％ (n=26), being significantly higher (p﹤0.001) than the mortality of patients without erythroblasts (3.0％). None of six patients with more than 2000 erythroblasts x 106/l survived. The postoperative detection of erythroblasts is highly predictive of death, the odds ratio after adjustment for the other known prognostic factors being 7.2 (95％ confidence interval 3.4–15.1). Erythroblasts were detected for the first time on average 11 ±2 days (median: 7 days; n=57) after surgery and 8 ±2 days (median: 6 days; n=26) before death. The detection of erythroblasts in blood after cardiothoracic surgery has a high prognostic significance in terms of in-hospital mortality, helping physicians to identify patients at high risk of death. This finding has to be confirmed by a prospective study with the use of a more sensitive and reliable technology and prospectively defined time intervals for counting blood cells.