Fifty-one renal allograft recipients (15-62 years old, mean: 37 years) were monitored for 2.5-6.5 years (average: 4.3 years) after surgery by using magnetic resonance imaging (MRI) to find (i) initial signs of osteonecrosis of the femoral head (ONF), (ii) the presence of bone marrow edema as an initial sign of ONF, (iii) any changes of MRI patterns, and (iv) the relationship between these MRI findings and prognosis. MRI was performed preoperatively (baseline), and whenever possible during the 6-9th week, 12-16th week, 12th month, and yearly thereafter. T1- and T2-weighted images were obtained by using a spin echo technique. Abnormalities were first detected on MRI of 23 femoral heads in 13 patients between 6 weeks and 12 months. All lesions first showed a low intensity band on T1-weighted images and a high intensity band on T2-weighted images. No symptoms or diffuse patterns, such as bone marrow edema, preceded the appearance of the band pattern. After the 12th month, no new abnormal findings on MRI were detected. The lesions were classified into Type A, B, or C, according to the location. 12 of the 16 Type C femoral head lesions, which extend beyond the medial two thirds of the weight-bearing portion of the acetabulum, became symptomatic 7-14 months after transplantation and then progressed to collapse. Bone marrow edema appeared with radiological collapse and symptoms. With the exception of five lesions in three patients who failed to be MR imaged until 12 months postoperatively, all lesions were first detected on MRI within 16 weeks after transplantation. We therefore postulate that the ischemic event that causes ONF will have occurred within 12 weeks after transplantation, considering the time lag of reparative reaction to the dead bone.