Three patients developed Q fever endocarditis on porcine bioprosthetic valves. They had a subacute or chronic course with nonspecific symptoms, enlargement of the liver and spleen, and cardiac failure due to destruction of the cusps, without disruption of the valve ring. High-phase I-specific IgG and IgA antibody titers against Coxiella burnetii were found. C. burnetii was isolated in each patient by inoculating suspensions of valve tissue into a human fetal diploid fibroblast cell line, which was grown as monolayers on slides contained inside rubber-stoppered tube cultures. Patients were treated successfully with doxycycline, cotrimoxazole, and valve replacement and were followed up for periods of 24 to 42 months; no evidence of deterioration was found. The human fetal diploid cell culture may be an expeditious, easy, and safe method to isolate C. burnetii from cardiac valves. Valve replacement seemed necessary to cure prosthetic-valve endocarditis due to C. burnetii infection. Combined therapy with doxycycline and cotrimoxazole may control the disease and prevent reinfection of the homografts replacing the valves.