Thirty-six femoral heads with non-traumatic osteonecrosis(ON) at precollapse stage in 32 patients were studied with repeated magnetic resonance(MR) imagings at three-month intervals to reveal the occurrence of marrow edema of the proximal femur in early stage ON. Eighteen hips were randomly selected for the core decompression and the remaining 18 hips were treated conservatively. The criteria of marrow edema in this study were signal abnormalities of low signal intensity in T1-weighted image, and isointensity or hyperintensity in T2-weighted image which involved the femoral head beyond the necrotic zone, extending to the neck and trochanteric area. Eight femoral heads demonstrated marrow edema at the initial MR imaging and seven out of eight were associated with pain for one to six weeks before the first MR study. Core decompression was performed on four out of those seven symptomatic femoral heads with marrow edema after the first MR imaging. The symptom was relieved promptly in three out of four core decompressed hips. The remaining one core decompressed case showed reccurrent marow edema on follow-up MR imagings. Out of 28 hips free of marrow edema at the initial MR study, 14 hips underwent core decompression after the first evaluation and the remaining 14 hips were treated conservatively. One out of 14 core-decompressed hips and three out of 14 nonoperated hips showed marrow edema in follow-up MR studies. Two cases experienced hip pain while two cases remained symptomless at the presence of marrow edema. Four femoral heads under-went core biopsy and three cases received total hip arthroplasty at the presence of marrow edema. In those four hips which underwent core decompression during the presence of marrow edema, intraosseous pressure(IOP) ranged from 36 to 60 (mean; 49) mmHg. In the remaining 14 core decompressed hips, the IOP ranged from 16 to 52 (mean; 37) mmHg. The histology of those four core biopsies and three resected femoral heads showed a collection of eosinophilic plasma-like fluid in the marrow space around the necrotic zone, a finding compatible with marrow edema. This study suggests that marrow edema of the proximal femur may occur during the progression of the ON of the femoral head and is frequently associated with increased intraosseous pressure and pain. Core decompression can relieve marrow edema and associated pain promptly. But the effect is temporary, and core decompression does not prevent marrow edema permanently.