Bartonella quintana (formerly Rochalimaea quintana) is a recently recognized cause of apparent "culture-negative" endocarditis. We describe a 39-year-old, homeless man who developed aortic valve endocarditis caused by B. quintana. He had a history of alcoholism and was seronegative for the human immunodeficiency virus. We established that B. quintana was the cause of the endocarditis on the basis of the isolation of B. quintana from blood cultures, the compatibility of histochemical stains of cardiac valve tissue, the reactivity of the polymerase chain reaction specific for B. quintana on cardiac valve tissue, and the failure to isolate an alternative causative organism despite extensive efforts. This is the second report of endocarditis caused by B. quintana and the fourth report of endocarditis caused by a Bartonella species. On the basis of the findings of this report and those of other recent reports, further study is warranted to determine the overall role of Bartonella species in apparent culture-negative endocarditis.