Cardiac valve pathology was evaluated in 15 patients with confirmed diagnosis of Bartonella endocarditis. Ten were infected by Bartonella quintana and 5 by Bartonella henselae. Histologic features of these cases, including fibrosis, calcification, vegetation, pattern of inflammation, and vascularization, were compared with those of valves from 25 cases of non-Bartonella endocarditis as controls using a computerized quantitative image analysis. Pathologic and immunohistologic testing for localization of Bartonella species in resected valves included Warthin-Starry stain and polyclonal antibody-based immunodetection. Compared with other cases of infective endocarditis, cases of Bartonella endocarditis are more fibrotic and calcified, less vascularized, with less extensive vegetation and chronic inflammation. These pathologic changes are suggestive of a prolonged infection. Warthin-Starry stain and immunohistologic testing demonstrated the presence of the organism, respectively, in 11 and 10 of the 13 tested valves. Results of both staining methods showed microorganisms in extracellular locations and in regions unaccompanied by inflammation. Pathology and immunohistology may contribute to the etiologic diagnosis of Bartonella endocarditis when serology and molecular techniques are not available.