Quantitative studies of femoral head blood flow by the hydrogen washout technique with the hips in various positions of immobilization were determined. Immobilization in flexion consistently resulted in the highest rate of femoral head blood flow. A position of relaxed abduction seemed to slightly, but significantly, decrease the rate of blood flow. The position of forced frog-leg abduction and forced internal rotation obliterated or drastically reduced circulation in the femoral head. We think that the ischemia of forced acute immobilization is secondary to compression of the immature cartilaginous femoral head. These experimental data further indicate the importance of the selected position of immobilization in the treatment of congenital dislocated hips as a cause of iatrogenic avascular necrosis.