Objective To evaluate the anatomical structure of bone in the formation of acetabular teardrop shadow. Methods The acetabular teardrop shadow of the X ray plain film in 100 children and 300 adults, and of CT in 43 cases were analysed 、measured and compared. Results In children, teardrop shadow appeared “U”in 80 00%, 14 00% in teardrop, 6 00% in line. In adult teardrop shadow appeared “U”in 41 33%, 53 33% in teardrop, 5 34% in line. There were significant differences between children and adult in the frequency of “U” and teardrop (? 2=43 34, P