Inflammation in atherosclerotic plaques makes them unstable and can cause thrombosis. Therefore, it is important to detect macrophage activity for clinical management of atherosclerosis. Peripheral benzodiazepine receptor (PBR) is expressed in various tissue and organs including macrophages. In this study, we tested whether inflammation characterized by macrophage infiltration can be detected by PBR binding. Six patients diagnosed as carotid atherosclerosis underwent endarterectomy. Using the fresh frozen sections, presence of PBRs and macrophages was examined by in vitro autoradiography using [(3)H]PK 11195 and immunohistochemical staining of CD68, respectively. All sections showed specific binding of [(3)H]PK 11195, and the staining with CD68 indicating macrophage infiltration. Density and distribution of PBR detected by [(3)H]PK 11195 autoradiography were consistent with those of the immunohistochemical staining. In conclusion, this study demonstrated that macrophage and inflammatory activity in atherosclerotic plaque can be imaged specifically by the binding of PBR indicating future application of PET imaging for PBR.