After aortic valve replacement for endocarditis, follow-up treatment with antibiotics is imperative. However, the question of how reliable preoperative and intraoperative diagnosis of endocarditis is in cases involving aortic defects is unclear. Of the 187 patients who underwent aortic valve replacement with or without coronary bypass surgery between June 1992 and June 1994, 150 exhibited no indications of endocarditis during preoperative and intraoperative examinations. In 17 cases (Group A) histological findings indicated acute florid endocarditis in 7 patients and chronic lymphocytic endocarditis in 10. Contrarily, histological examinations of 133 patients (Group B) revealed myxoid and/or sclerotic valve degeneration. WBC and LDH activity, examined one day preoperatively and on the first and second days postoperatively, exhibited no significant differences between the two groups, with the exception of LDH activity on the first postoperative day (Group A: 490 +/- 114, Group B: 403 +/- 132, p = 0.04). Of the clinically asymptomatic patients requiring aortic valve replacement, 11.3% exhibited acute florid endocarditis upon histologic examinations. This subgroup cannot be identified based upon routine preoperative or postoperative laboratory tests or intraoperative observation. Histological examination of the aortic valve is useful for identifying the high percentage of otherwise nonidentifiable endocarditis. Further study will be required to determine therapeutic recommendations based upon such diagnosis.