Introduction : The incidence of culture-negative infective endocarditis appears to be high in Egypt. This is conventionally attributed to the indiscriminate use of antibiotics. Application of serologic tests and tissue microbiology might facilitate the detection of the organism. Patients and Methods : We prospectively followed up 139 patients with suspected infective endocarditis referred to Kasr El Eini hospitals from February 2005 to February 2007. Modified Duke’s criteria were used for case definition. Serologic testing was performed for Coxiella burnetii,Bartonella, Brucella, Chlamydia, Mycoplasma, Legionella and Aspergillus antibodies in the sera of all patients. Resected surgical material was cultured whenever available. Results : One hundred and five patients had definite or possible IE; 53 of them were culture-negative(50.5%). Fourty three of the culture-negative patients reported previous antibiotic intake.Application of serologic tests identified the causative organism in 12 out of 53 culture-negative endocarditis patients (22.6%). Culture of the surgically excised material identified the organism in 8 patients (15.1%). Thus, the proportion of patients with no identified organism dropped from 50.5% to 35.2%. Conclusion: This series reveals a high incidence of culture negative endocarditis in Egypt. The most common association is prior antibiotic intake. The proportion of unidentified organisms dropped from 50.5% to 35.2% by the application of serologic tests and tissue microbiology. This study emphasizes the importance of serologic testing and tissue microbiology in the diagnostic workup of infective endocarditis.