Culture negative infective endocarditis (CNE) poses very difficult problems during treatment. In this study it was found that of 132 surgically treated patients with infective endocarditis, causative organism was not identified in 46 (34.8 %). Pre- and perioperative conditions and clinical results of these patients were evaluated. CNE remained very frequent even in these years, and it did not decrease with time. Antibiotic treatment prior to microbiological examinations was commonly observed (nearly 90% orally, and 70% intravenously). In average, it took more than 2 months to establish the diagnosis of CNE after the onset, and both aortic and mitral valves were affected frequently (19.0 %). New York Heart Association functional class IV was observed significantly more commonly (61.9%) than culture positive patients. Frequencies of prosthetic valve endocarditis (12.2%), periannular abscess (36.3%), and embolism (21.4%) were similar. Infection was fairly controllable before surgery in 43.9% of CNE patients and in-hospital mortality rate was 14.3%, both of which were comparable to those of all culture positive patients. However, recurrence rate was relatively higher (10.0%). The conditions and outcomes of CNE were comparable to Staphylococcal endocarditis in some aspects, and were relatively worse than overall culture positive endocarditis.