We used magnetic resonance imaging (MRI) to evaluate retrospectively 32 hips with avascular necrosis of the femoral head before and after core decompression and bone grafting. At a median follow-up time of 15 months, 4 of 9 large lesions had undergone femoral head collapse; 2 small lesions had decreased in size; and 14 small, 6 moderate, and 5 large lesions were unchanged. One hip with biopsy-proven avascular necrosis had diffuse marrow edema which resolved after surgery. The signal pattern within the lesions was analyzed in 17 hips. Fifteen lesions were unchanged, while one fat-like lesion became fibrous-like and one edema-like lesion resolved. Analysis of the femoral neck marrow signal in one patient revealed that premature fatty marrow conversion had reverted to hemopoietic marrow 2 years after surgery. Most lesions that appeared stable on MRI were clinically stable or improved. MRI can demonstrated changes in size and signal characteristics as well as femoral head collapse after core decompression and bone grafting. Changes in the surrounding marrow signal, including resolution of marrow edema and reconversion from fatty to hemopoietic marrow, can also be detected.