Objective To investigate the mid-and long-term effects of metal-on-metal hip resurfacing arthroplasty (HRA),and to analyze its related factors.Methods A total of 64 patients (81 hips) who underwent metal-on-metal HRA from June 2005 to January 2013 were recruited in the present study.There were 35 males (44 hips) and 29 females (37 hips) with the mean age of 48.26±10.45 years (20-65 years),including 47 unilateral and 17 bilateral HRAs.The cohort consisted of osteoarthritis secondary to the developmental dysplasia of the hip (23 cases,29 hips),necrosis of the femoral head (19 cases,22 hips),osteoarthritis (8 cases,10 hips),rheumatoid arthritis (5 cases,9 hips),ankylosing spondylitis (6 cases,8 hips),pigmented villonodular synovitis (2 cases,2 hips) and Otto's disease (1 case,1 hip).During the follow-up duration,radiographic features,including acetabular inclination angle,stem-femoral shaft angle,component loosening,osteolysis,femoral neck narrowing and heterotopic ossification,were evaluated by hip X-rays in straight and froglike position.The size and type of pseudotumor were assessed by MRI and ultrasonography.Clinical efficacy was evaluated by Harris hip score and the University of California at Los Angeles (UCLA) hip score.Considering revision surgery as the end point,the component survivorship was calculated.Results The mean follow-up was 7.98±2.21 years,ranging from 5.0 to 12.8 years.The mean postoperative Harris hip score (92.01 ±5.69) was higher than the preoperative score (41.93 ±9.09).The mean postoperative UCLA pain,walking,function,activity scores (9.37±0.86,9.14± 1.01,8.77± 1.09,6.47± 1.27,respectively) were improved when compared with the preoperative UCLA scores (3.57± 1.23,5.99± 1.30,5.00± 1.01,3.84± 1.41,respectively).The postoperative flexion,abduction and adduction,medial and lateral rotation of the hip was larger than the preoperative ones.Complications occurred in 10 hips (12.3％,10/81).Seven patients (8 hips) experienced early and intermediate complications,including one intraoperative femoral nerve injury,one deep femoral artery and saphenous nerve injuries during the same surgery,one unexplained pain of hip,one femoral neck fracture,three hips of heterotopic ossification,and one pseudotumour.There were mid-and long-term complications in two hips,including one narrowing of the neck and one pseudotumour which was occurred at 9 years.There was one patient (2 hips) underwent revision surgery twice at 5 months and 9 years.The former cause of revision was femoral neck fracture and the latter one was pseudotumour.The Kaplan-Meier survivorship was 98.8％ at five years,and 95.0％ at ten years.Conclusion Patients who underwent metal-on-metal HRA could obtain good mid and long-term results.Pseudotumour and unexplained pain of the hip are critical factors which can affect the mid-and long-term results and survivorship of metal-on-metal HRA.