The incidence and clinical and magnetic resonance imaging features of osteonecrosis of the hip were evaluated in patients with aplastic anemia. Two hundred and forty-one patients with aplastic anemia were examined using MR imaging of bone marrow during the five years from 1994 to 1998. Osteonecrosis of the hip was observed on MR imaging in nineteen (15 males and 4 females, mean age 35 yr) of the 241 patients. It was present in both hips in 14 patients, and there were five cases with unilateral occurrence, with a total of 33 involved hips. All except for five hips with associated bone marrow edema revealed increased fatty marrow conversion in the proximal femoral metaphysis. In nine patients, osteonecrosis was detected without any pain. Five patients already had osteonecrosis before any medication was administered. Twelve patients received antilymphocyte globulin, and seven patients received a low dose of steroids before the MR diagnosis of osteonecrosis. Osteonecrosis of the hip frequently develops in patients with aplastic anemia (7.9%), associated with fatty marrow conversion of the proximal femoral metaphysis.