Q fever endocarditis is a chronic disease with protean manifestations. The clinical and serological manifestations of nine patients diagnosed as having Q fever endocarditis during a 19-year period are reviewed. Four patients (44%) required valve replacement due to congestive heart failure. Three of these four patients were diagnosed as having Q fever endocarditis only after elective valve surgery, by histopathological examination of the valve and subsequent serological tests. Prior to surgery they were afebrile and had no other symptom or sign indicative of endocarditis. The antibiotic treatment and the decreasing titres of Q fever antibodies of all nine patients during several years of follow-up are summarized. Careful assessment of heart valves for histopathological evidence of inflammation is suggested, even after elective replacement. If found, clinical and laboratory evaluation should include determination of anti-Coxiella burnetti antibodies.