The purpose of this work is to evaluate the performance of a computer based analysis system aimed at the quantitative detection of changes in hip osteolytic lesions in subjects with hip implants. The computer system is based on the supervised segmentation of a baseline x-ray computed-tomography (CT) scan and an automated segmentation of a follow-up CT scan using an object based tracking algorithm. The segmentation process outlines the pelvic bone and lesions present in the pelvis. The size and CT density of the osteolytic lesions are computed in both baseline and follow-up segmentations and the change in both these quantities are evaluated. The system analysis consisted of the direct comparison of the quantitative results obtained from an expert manual segmentation to the quantitative results obtained using the automated system on 20 subjects. The system bias was evaluated by performing forwards and backwards analysis of the CT data. Furthermore, the stability of the proposed tracking system was compared to the variability of the manual tracking. The results show that the system enhances the human ability to detect changes in lesions size and density regardless of the inherent observer variability in the definition of the baseline manual segmentation.