A prospective study was undertaken to determine the diagnostic accuracy of magnetic resonance imaging in the evaluation of a symptomatic hip for which a diagnosis of early ischemic necrosis of the femoral head was suspected. Fifteen patients (sixteen symptomatic hips), for whom the findings of magnetic resonance imaging were consistent with a diagnosis of osteonecrosis of the femoral head, had a core decompression and a biopsy of the contents of the core. Preoperative magnetic-resonance imaging was useful for planning which segment of the femoral head should be biopsied. Plain radiographs and tomograms of the hips were also made. On the basis of the plain radiographs, ten hips were determined to have Stage-I findings and six hips, Stage-II ischemic necrosis, according to the system of Ficat and of Arlet and Ficat. Histological study revealed evidence of necrosis in all of the biopsy specimens of bone. We concluded that findings of magnetic resonance imaging that are characteristic of osteonecrosis correlate well with the results of biopsies of bone in patients who have an early stage of ischemic necrosis. Magnetic resonance imaging is a highly sensitive and specific method for both the diagnosis and the location of Stage-I and Stage-II osteonecrosis.