A carefull inspection of an AP X-ray of a normal hip reveals in the weight-bearing surface of the ilium a curved area of dense bone, called SOURCIL by Pauwels (1963). The configuration of this dense area is semilunar and represents subchnodral bony eburnation due to a respose by the articular portion of the ilium to the stress provoked by the compressive force acting on it. However, in many cases of idiopathic avascular necrosis of the femoral head, changes in SOURCIL may be seen. So we reviewed our 49 patients (unilateral 16, bilateral 33) since 1983 with comparison to normal hips (100 persons). The results are as follows; 1. SOURCIL is not visible at birth and by walking, it is developed with age and has the adult configuration at the age of 18. It is thought the configuration and sloping angle of SOURCIL is not changed with age after skeletal growth is completed. 2. Although the position of the X-ray tube may be changed, the shape and angle of SOURCIL is not changed significantly. 3. The average SOURCIL angle is 2.1° in normal hip group and 3.5° for the avascular necrosis group but, there is no meaningful statistical differences. It seems to be meaningful that the mean SOURCIL angle is 10.3° for patients under the age of 35 in avascular necrosis groups (9 persons). 4. If a high SOURCIL angle (about 10° or more) is present, the hemiarthroplasty such as bipolar endoprosthesis should be avoided because there is biomechanically far more wearing of the acetabular roof after that kind of operation. If inevitable, deepening the acetabulum by reaming is recommended for prevention of superolateral migration of prosthesis after operation. 5. With close observation of SOURCIL before and after operation, one can easily point out what is able to predict the prognosis on the biomechanical basis.