Q fever (Coxiella burnetii) is an uncommon cause of culture-negative infective endocarditis. Four cases of Q fever endocarditis diagnosed at our hospital in the last 7 years are reported (8% of all infective endocarditis). Infection involved a prosthetic heart valve in 3 cases (15% of all prosthetic valve endocarditis), and one patient with single ventricle and pulmonary stenosis in the remaining case. Important complications developed in all cases, and 3 patients underwent cardiac surgery. Mortality was 50%. Some diagnostic and therapeutical aspects of this disease are discussed.