Thirty-two hips affected by aseptic necrosis, four of them post-traumatic and 12 contralateral hips thought to be healthy have been studied by combining superselective arteriography with the injection of radioactive microspheres. It has been possible to show that aseptic necrosis begins with a global ischaemia and is followed by an incomplete revascularisation leaving a necrotic area. On the border between the two areas hypervascularity produces a zone of fragility where microfractures develop with detachment of a sequestrum. The presumed opposite hip is almost always, even in traumatic lesions, the site of a hypovascularity which seems to suggest a predisposition to the lesion. There is a correlation between radiological and clinical worsening and the development of revascularisation in the femoral head.