OBJECTIVE To investigate whether levosimendan diminishes the incidence of heart failure after cardiac surgery. METHODS Prospective, randomized, placebo-controlled clinical study. METHODS Cardiac surgery operating room and postanesthesia care unit in a university hospital. METHODS Two hundred patients assigned to undergo heart valve or combined heart valve and coronary artery bypass grafting surgery. METHODS Patients were randomized to receive a 24-hr infusion of placebo or levosimendan administered as a 24 μg/kg bolus over 30-mins and thereafter at a dose of 0.2 μg/kg/min. RESULTS Heart failure was defined as cardiac index<2.0 L/min/m or failure to wean from cardiopulmonary bypass necessitating inotrope administration for at least 2 hrs postoperatively. Heart failure was less frequent in the levosimendan compared to the placebo group: 15 patients (15%) in the levosimendan and 59 patients (58%) in the placebo group experienced heart failure postoperatively (risk ratio 0.26; 95% confidence interval 0.16-0.43; p<.001). Accordingly, a rescue inotrope (adrenaline) was needed less frequently in the levosimendan compared to the placebo group (risk ratio 0.11; 95% confidence interval 0.01-0.89), p=.005. Intra-aortic balloon pump was utilized in one patient (1%) in the levosimendan and in nine patients (9%) in the placebo group (risk ratio 0.11; 95% confidence interval 0.01-0.87), p=.018. The hospital and the 6-month mortality were comparable between groups. There were no significant differences in major organ failures postoperatively. Eighty-three patients were hypotensive and needed noradrenaline in the levosimendan compared to 52 patients in the placebo group, p<.001. The cardiac enzymes (creatine kinase MB isoenzyme mass) indicating myocardial damage were lower in the levosimendan group on the first postoperative day, p=.011. CONCLUSIONS In the present study, levosimendan infusion reduced the incidence of heart failure in cardiac surgery patients but was associated with arterial hypotension and increased requirement of vasopressor agents postoperatively. Improved mortality or morbidity was not demonstrated.