Objective To provide the objective basis for the evaluation of the operative results of vascularized greater trochanter bone flap in treating osteonecrosis of the femoral head (ONFH) by three-dimensional gait analysis. Methods Between March 2006 and March 2007, 35 patients with ONFH were treated with vascularized greater trochanter bone flap, and gait analysis was made by using three-dimensional gait analysis system before operation and at 1, 2 years afteroperation. There were 23 males and 12 females, aged 21-52 years (mean, 35.2 years), including 8 cases of steroid-induced, 7 cases of traumatic, 6 cases of alcohol ic, and 14 cases of idiopathic ONFH. The left side was involved in 15 cases, and right side in 20 cases. According to Association Research Circulation Osseous (ARCO) classification, all patients were diagnosed as having femoral-head necrosis at stage III. Preoperative Harris hip functional score (HHS) was 56.2 ± 5.6. The disease duration was 1.5-18.6 years (mean, 5.2 years). Results All incisions healed at stage I without early postoperative compl ications of deep vein thrombosis and infections of incision. Thirty-five patients were followed up 2-3 years with an average of 2.5 years. At 2 years after operation, the HHS score was 85.8 ± 4.1, showing significant difference when compared with the preoperative score (t=23.200, P=0.000). Before operation, patients showed a hip muscles gait, short gait, reduce pain gait, and the pathological gaits significantly improved at 1 year after operation. At 1 year and 2 years after operation, step frequency, pace, step length and hip flexion, hip extension, knee flexion, ankle flexion were significantly improved (P lt; 0.01). Acceleration-time curves showed that negative wave and spinous wave at acceleration-stance phase of front feet and hind feet in affected l imb were obviously reduced at 1 year and 2 years after operation. Postoperative petronas wave appeared at swing phase; the preoperative situation was three normal phase waves. Conclusion These results suggest that three-dimensional gait analysis before and after vascularized greater trochanter for ONFH can evaluate precisely hip vitodynamics variation.