It is possible to create a diastasis in the hip by traction. The space thus created between articular surfaces can be visualized radiographically and is known as “a vacuum phenomenon”. Twenty one cases of unilateral hip joint disease were studied with vacuum arthrography from Oct. 1983 to July 1984 in Catholic Medical College and Center. The patients ages ranged from a month to 53 years of age but most of them(18 out of 21 patients) were below 10. There were 5 female patient's and 16 male patients in this series. The results were as follows: 1. In the absence of an effusion in all normal hip joints the vacuum phenomenon was demonstrated. 2. In the absence of an effusion in the hips of a transient synovitis, 4 Legg-Calve-Perthes disease and 3 sprain the size of vacuum was normal. 3. The size of vacuum of 3 transient synovitis and a pyohip of healing stage which had only mild effusion was smaller than those of normal hips. 4. In the presence of large amount of effusion in the hips of 5 transient synovitis, 2 pyogenic arthritis and 2 tuberculous arthritis it has not been possible to produce the vacuum phenomenon. 5. This study, in summary, shows that the size of vacuum represents the amount of effusion. 6. Non-ossified femoral head in the new born, articular surface of acetabulum and thickness & local changes in the cartilage of the femoral head could be demonstrated using the vacuum arthrography. 7. It has not been possible to demonstrate the articular surface lateral to the limbus, ligamentum teres and limbus in the vacuum arthrogram.