The pathogenesis of corticosteroid-induced femoral head necrosis is assumed to be ischemia. The purpose of this study was to investigate the perfusion pattern of the femoral head and plasma coagulability during 24 h corticosteroid megadose treatment, as recommended by the National Acute Spinal Cord Injury Studies (NASCIS), in the awake big animal model. Blindedly, 9 animals underwent megadose methylprednisolone infusion (30 mg/kg intravenously as a n initial bolus, followed by 5.4 mglkglh for further 23 h) while 9 animals served as placebo treated controls. Regional blood flow of the systematically subdivided femoral head, proximal femur, acetabulum, and soft tissue hip regions was investigated by the microsphere technique at steady state (phase I), after the initial bolus infusion (phase 2), and after the completed treatment (phase 3). Plasma coagulability was examined in phases 1 and 3. Blood flow of the femoral head epiphysis and metaphyseal cancellous bone was unchanged after one hour of steroid infusion, but decreased after the completed treatment at 24 h in the experimental group. Femoral head blood flow reduction was global without a tendency to more pronounced blood flow decrease in any subregion. Plasma fibrinogen was significantly higher after 24 h of steroid infusion than in the placebo control group. 24 h high dose methylprednisolone treatment causes femoral head blood flow reduction and hypercoagulability of plasma in the normal awake immature pig. These findings may be pathogenetic factors in the early stage of steroid-induced osteonecrosis.