The pathology of the posterior acetabular legion in femoroacetabular impingement (FAI) syndrome, so called “contre-coup region”, is still unclear. . 18. F-fluoride positron emission tomography (PET) is a functional imaging modality, which reflects the osteoblast activity. Recent technological advances in PET combined with computed tomography (CT) imaging allowed us to obtain detailed 3-dimensional (3D) morphological information. We evaluated the abnormal uptake of . 18. F-fluoride PET/CT on posterior acetabular lesion in FAI syndrome cases. We enrolled forty-one hips from 41 patients who were diagnosed as FAI syndrome and were performed . 18. F-fluoride PET/CT between October 2014 and October 2016. In each hip, the maximum standardized uptake value (SUV. max. ) on the posterior acetabular was measured. The cases were divided into 4 groups; cam-type (11 cases), pincer-type (7), combined-type (11), dysplastic developmental hip (DDH) with cam morphology (12). The average SUV. max. of the pincer-type was significantly smaller than that of the other 3 groups (p < .05). The percentage of the cases with SUV. max. ≥ 6 was 81.8% in cam-type, 28.6% in pincer-type, 90.9% in combined-type, 91.7% in DDH with cam morphology. Furthermore, the average degree of α angle of the cases of SUV. max. ≥ 6 was significantly higher than that of the cases of SUV. max. < 6 (p = .005). Although actual biomechanical mechanism in contre-coup region is still controversial, this result indicated that the cam morphology related to the posterior acetabular lesion with accelerated bone metabolism