A patient with Q fever endocarditis, which is almost unknown in the United States, was followed for a total of 32 months; the study was begun 3½ months before aortic valve replacement. Diagnosis was confirmed by serology, visualization of Coxiella burnetii in excised aortic valve tissue by direct and immunofluorescence staining, and isolation of C. burnetii from aortic valve tissue. Serum antibodies against phase I and phase II antigens of C. burnetii were identified. Almost all phase I and phase II antibodies were IgG. These findings are compared with those in an uncomplicated case of acute Q fever. New findings on the immune response to chronic Q fever are presented.