Background Several animal studies suggest beneficial effects on kidney function upon administration of levosimendan. As recent data from clinical studies are heterogeneous, we sought to investigate whether levosimendan is associated with improved postoperative kidney function in cardiac surgery patients with respect to timing of its administration. Methods Retrospective, single centre, observational analysis at a university hospital in Berlin, Germany. All adult patients without preoperative renal dysfunction that underwent coronary artery bypass grafting and/or valve reconstruction/replacement between 01/01/2007 and 31/12/2011 were considered for analyses. Results Out of 1.095 included patients, 46 patients were treated with levosimendan due to a severely reduced left ventricular systolic function preoperatively (LVEF < 35%) and/or clinical signs of a low cardiac output syndrome. Sixty-one percent received the drug whilst in the OR, 39% after postoperative intensive care unit admission. When levosimendan was given immediately after anaesthesia induction, creatinine plasma levels (p = 0.009 for nonparametric analysis of longitudinal data in a two-factorial design) and incidence of postoperative renal dysfunction (67.9% vs. 94.4%; p = 0.033) were significantly reduced in contrast to a later start of treatment. In addition, duration of renal replacement therapy was significantly shorter (79 [35;332] vs. 272 [132;703] minutes; p = 0.046) in that group. Conclusions Postoperative kidney dysfunction is a common condition in patients under going cardiac surgery. Patients with severely reduced left ventricular function and/or clinical signs of a low cardiac output syndrome who preoperatively presented with a normal kidney function may benefit from an early start of levosimendan administration, i.e. immediately after anaesthesia.