(1) Infective endocarditis is a severe inflammatory disease associated with substantial mortality and morbidity. Alkaline phosphatase (AP) levels have been shown to change significantly during sepsis additionally we previously found higher initial AP drop after cardiac surgery is associat-ed with unfavorable outcomes. Therefore, the course of AP after surgery for endocarditis is of special interest. (2) 314 patients with active isolated left-sided infective endocarditis at the De-partment of Cardiac Surgery (Medical University of Vienna, Austria) between 2009 and 2018 were enrolled in this retrospective analysis. Blood samples were analyzed at different time points (baseline, postoperative day 1-7, postoperative days 14, and 30). Patients were categorized ac-cording to relative alkaline phosphatase drop (≥30% vs. <30%). (3) The rate of postoperative renal replacement therapy with or without prior renal replacement therapy (7.4 vs. 21.8%; p=0.001 and 6.7 vs. 15.6%; p=0.015, respectively) and extracorporeal membrane oxygenation (2.2 vs. 19.0%; p=0.000) was observed after a higher initial alkaline phosphatase drop. Short-term (30-day mor-tality 3.0 vs. 10.6%; p=0.010) and long-term mortality (p=0.008) were significantly impaired after a higher initial alkaline phosphatase drop. (4) The higher initial alkaline phosphatase drop was accompanied by impaired short and long-term outcomes after cardiac surgery for endocarditis. Future risk assessment scores for cardiac surgery should consider alkaline phosphatase.