Fifty-six open hip operations of different types afforded an opportunity to study the blood supply to the femoral head. In forty-four instances the ligamentum teres was cut; 70 per cent bled actively. Twenty-four capital fragments were examined in situ for the presence of active bleeding from their cancellous surfaces. Four were avascular, twelve bled sluggishly, and eight bled actively. The eight heads were studied for the effects of various malpositions on their bleeding activity. Rotatory malposition, either anterior or posterior, interrupted active bleeding from the head fragments. Of the non-rotatory malpositions, only the extreme valgus position consistently interrupted bleeding. Related anatomical studies were carried out on fresh cadaver specimens to determine the effects of normal and abnormal ranges of femoral-head movement on the ligamentum teres and its foveal vessels. The conclusions are: 1. In the intact hip it is impossible for the ligamentum teres to become sufficiently twisted, stretched, or compressed to interrupt its supply of blood to the femoral head. 2. In the fractured hip abnormal degrees of rotation of the free femoral fragment can easily result in twisting, stretching, and compression of the ligamentum teres to the point of complete interruption of its blood supply. 3. Rotatory malposition may be present and yet not be noticed in an otherwise anatomically reduced fracture. 4. Extreme valgus position interrupts foveal blood supply and should be avoided. 5. Fixation of subcapital fractures in either rotatory malposition or in extreme valgus position could account for many instances of delayed union, nonunion, aseptic necrosis, and degenerative arthritis. If this is so, detection and correction of thsese malpositions should be added to our criteria of management.