Background. The aim of this study was to investigate the diagnostic values of serum procalcitonin (PCT) and C-reactive protein (CRP) levels in infective endocarditis (IE) and to correlate them with the etiology of the disease and the prognosis of the patients. Methods: Fifty patients who were diagnosed as having IE based on Duke criteria (major and/or minor) were included in the study at the Istanbul University Cardiology Institute and Florence Nightingale Hospital. Forty patients with bacteremia (non-IE) and 50 healthy blood donors were also included in the study as the control group. During the 45 days of medical follow-up, in those patients who had a response to medical therapy based on the results of left ventricular function tests, transesophageal echocardiography (TEE) and culture, among other factors, PCT and CRP levels were measured in 5-cm(3) blood samples obtained without anticoagulant when they were first admitted (day 0), as well as 24 h and 15, 30 and 45 days after admission. In the patients who had valve replacement, 5-cm(3) blood samples without anticoagulant were similarly obtained on the day of admission, after 24 h and/or on the 15th day, and 1 day before and on the 2nd and 5th days after the operation. Results: In this study, a significant difference (p < 0.001) was found between the IE group and the healthy control group with respect to their serum PCT and CRP levels at the time of admission. No significant difference was found between IE and non-IE groups (p > 0.05). The sensitivity of PCT in comparison to CRP was found to be lower (84 vs. 100%); however, its specificity was determined to be higher (88 vs. 72%). The median values of serum PCT in the nonoperated and operated cases at the time of admission, after 24 h and on the 15th day were 3.71, 5.35 and 0.44, and 2.45, 4.28 and 4.22 ng/ml, respectively, and those of CRP were 9.30, 10.95 and 10.65, and 9.5, 10.9 and 10.2 mg/dl, respectively. The median values of serum PCT were found to be higher in cases with IE and non-IE related to gram-negative bacteria than those related to gram-positive bacteria (p < 0.02). This was found to be insignificant for CRP (p > 0.05). Conclusions: As a result, this study suggests that in the diagnosis of IE, it would be beneficial to use PCT, besides TEE, culture and other clinical criteria, for its high specificity and positive predictive value in comparison to CRP. This study also suggests that in determining the response to medical treatment in the follow-up period, PCT could be a more valuable parameter than CRP, as PCT has a high prognostic value and is a good indicator for valve replacement in addition to the major criteria. Furthermore, serum PCT levels may help the physician to decide on the antimicrobial therapy combination before obtaining the culture results, or in situations in which the agent could not be isolated yet. Copyright (C) 2003 S. Karger AG, Basel.