Bartonella quintana may cause trench fever, endocarditis, bacillary angiomatosis, and chronic bacteremia, and a reemergence among homeless populations in cities has been noted. Pericarditis from Rickettsia conorii and Coxiella burnetii infection has been described, but there have been no reports of pericarditis due to Bartonella spp. We report a case of pericardial effusion due to Bartonella quintana in a homeless man, diagnosed on the basis of PCR detection of Bartonella quintana in a pericardial biopsy sample and a fourfold rise in antibody titers. The patient recovered within 2 weeks with antibiotics active against bartonellae.