Bartonella henselae and B. quintana induce an unusual vascular proliferative tissue response known as bacillary angiomatosis (BA) and bacillary peliosis (BP) in some human hosts. The mechanisms of Bartonella-associated vascular proliferation remain unclear. Although host factors probably play a role, microbial coinfection has not been ruled out. Because of the vascular proliferative characteristics noted in both Kaposi's sarcoma (KS) and BA and occasional colocalization of KS and BA, the possibility was explored that KS-associated herpesvirus (KSHV) might be associated with BA lesions. Tissues with BA and positive and negative control tissues were tested for the presence of KSHV DNA by a sensitive polymerase chain reaction assay. Only 1 of 10 BA tissues, a splenic biopsy, was positive in this assay; this tissue was from a patient with concomitant KS of the skin. Thus, KSHV is probably not involved in the vascular proliferative response seen in BA-BP.