Coxiella Burnetii endocarditis is very rare. It is the main complication of the chronic form of Q fever. Blood cultures are negative and clinical presentation very variable and diagnosis is essentially based on indirect immunofluorescence serum analysis. The authors report the case of a 19 year old patient with a history of rheumatic aortic regurgitation admitted for an episode of left ventricular failure in a context of long-term pyrexia without valvular vegetations or mutilation. The antiphase I Ig G antibody levels were significant. Treatment with doxycycline and fluoroquinolone was initiated. The clinical improvement was spectacular. Three months later, the patient underwent aortic valve replacement and histological examination of the valve showed subacute endocarditis on chronically fibrotic valvular disease. This is an interesting case by its rarity and its diagnostic and therapeutic problems.