Core decompression is still widely used in avascular necrosis of the femoral head but the results are unpredictable and the indications are not settled yet. The reparative process of the decompressed femoral head is poorly understood. Seven cases in 5 patients were undertaken THRA following failed decompression and these were studied for the radiological and pathological changes of the core tracts. The lesions of failed cases were involved more than 1/3 of femoral head on MRI and all cases were stage II A or B. The extent of the necrotic area in MRI was enlarged with crack, sclerosis and sometimes gas collection. Depression of the subchondral plates were also observed. Capillary ingrowth or neovascularization was not found at all and there were only fibrosis, inflammatory response and foreign body reactions.