We found that a painful hip without radiological changes presented a modification of the magnetic resonance image of the femoral head and neck with diffuse low T1 weighted and high T2 weighted signal. Core biopsy showed that the bone marrow was replaced from the neck to the head of the femur by fibroblastic tissue and edema without detectable trabecular bone impairment. Classical radiographic, CT, and MRI abnormalities of osteonecrosis of the femoral head were present 14 months later. This report suggested that diffuse femoral head and neck medullary impairment with fat cell replacement by fibroblastic tissue without evidence of trabecular bone necrosis can be found at the early stages of idiopathic osteonecrosis.