We describe an immunocompromised renal transplantation patient with opportunistic lung infection due to Bartonella henselae (formerly Rochalimaea henselae) and provide evidence suggesting transmission from a pet cat. Computed tomographic scans of the chest and lung biopsies provided material for diagnosis. The etiology was established by polymerase chain reaction and sequencing of a 16S ribosomal DNA segment from infected lung tissue. Histopathologic and serological evidence supported the molecular data. B. henselae was isolated from the blood of eight of the patient's many cats. The patient responded to prolonged therapy with doxycycline, and relapse did not occur during a 1-year follow-up. B. henselae joins a long list of pathogens that can cause lung infections in association with cell-mediated immunodeficiency states. Molecular methods are useful in diagnosis of this infection in light of the bacterium's fastidious growth characteristics. If an immunocompromised patient has lung nodules and a history of exposure to cats, B. henselae should be sought in biopsy specimens.