PURPOSE: To predict early risk of osteonecrosis of the femoral head by comparison of the bone marrow pattern of the proximal femoral metaphysis(PFM) in normal subjects and patients with osteonecrosis of the femoral head on T1-weighted magnetic resonance (MR) images. MATERIALS AND METHODS: The authors retrospectively reviewed T1 (TR525/TE 25 msec) weighted coronal MR images of 67 hips with osteonecrosis and 65 normal hips in 39 patients with osteonecrosis of the femoral head and in 27 normal subjects. On the basis of bright signal intensity of fat, the proportion of remaining hematopoietic marrow in PFM was subdivided into 4 grades (0 to 3) by two radiologists. No evidence of remaining hematopoietic marrow was assigned grade 0, and grades 1, 2 and 3 represented scanty, moderate, and prominent hematopoietic marrow, respectively. Grades 0 and 1 were collectively defined as "predominantly fatty", grades 2 and 3 as "predominantly hematopoietic". The frequency of the predominantly fatty marrow in PFM was analyzed in relation to three age groups(<25,25-50,50<) and both sexes. RESULTS: The overall frequency of predominantly fatty marro in PFM was higher in hips with osteonecrosis than in normal hips (p<0.001). Especially in the male population under the age of 50, the frequency was apparently higher in hips with osteonecrosis, compared with normal hips (p<0.001). However, the male population aged over 50 or female population showed no statistically significant difference in our series. CONCLUSION: In proximal femoral metaphysis with osteonecrosis of the femoral head, fatty marrow conversion occurs apparently earlier than in normal subject. T1-weighted MR imaging could therefore be useful in predictiong early risk of osteonecrosis of the femoral head because of early fatty marrow conversion of the proximal femoral metaphysis.