Objective To study the clinical efficacy of platelet-rich plasma (PRP) in the treatment of knee osteoarthritis (KOA) and evaluate whether the age,body mass index and grade of KOA are associated with the treatment outcomes.Methods Using the prospective,randomized,controlled study,100 KOA patients hospitalized between December 2013 and November 2014 were enrolled.Twentyeight patients were men and 72 were women.Mean age was 58 years (range,35-85 years).Degenerative arthritis occurred in 68 patients and traumatic arthritis in 32 patients.Kellgren-Lawrence (K-L) score was grade Ⅱ in 35 patients,grade Ⅱ in 46 and grade Ⅲ in 19.The patients were assigned to receive hyaluronic acid (HA) (HA group,n =50) and PRP (PRP group,n =50) by an intraarticular route once weekly for 3 weeks,according to the random number table.Between-group differences were insignificant in age,gender,body mass index (BMI) and K-L grade.Western Ontario and McMaster Universities Arthritis Index (WOMAC),visual analog scale (VAS) and cartilage lesions score (CaLs) were used for clinical and MRI evaluations.At follow-up evaluation,the effective rate was defined at least 36％ improvement from the baseline WOMAC score.Results All patients were followed up for 6 months.The effective rate in PRP group was 84％ versus 68％ in HA group after the last treatment (P ＞0.05),and was 60％ versus 36％ in HA group at the final follow-up (P ＜ 0.05).WOMAC score in both groups had significant improvement after operation,while VAS improved only in PRP group (P ＜ 0.01).In PRP group patients with K-L grade I had better VAS and WOMAC scores than those with grade Ⅱ (P ＜0.05),and patients with grade Ⅱ had better WOAMC score than those with grade Ⅲ (P ＜ 0.05).MRI findings showed seven patients in PRP group had similar CaLs before and after operation (P ＞ 0.05),and the area of abnormal signal in subchondral bone and the depth of cartilage lesion gradually decreased in one of them.Follow-up study showed the outcomes had negative correlation with age and K-L grade (P ＜0.05),but no certain correlation with BMI in PRP group (P ＞ 0.05).Clinical effects in both groups were decreased over time.Conclusions Intraarticular injection of PRP benefits to pain relief,decreased inflammation and tissue repair,and has much better outcome in patients with younger age and lower K-L grade.However,BMI is not associated with the outcome.