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Background The range of motion after a total knee arthroplasty is an important clinical outcome affecting the life of the patient. The aim of this study was to determine the most important factors influencing the postoperative knee flexion in Tehran, Iran. Methods Between July 2007 and January 2009, on 95 cases of total knee joint replacement (89 patients), who were followed for 1 year postoperatively, the risk factors were assessed. Patient demographics (sex, age, body mass index, previous surgery, preoperative Knee Society System score, and preoperative range of motion) as well as radiographic measurement for preoperative tibiofemoral angle were statistically analyzed and the probable predictors entered into a linear regression model. Results Univariate analysis showed that age, preoperative flexion angle, preoperative flexion arc and preoperative tibiofemoral angle had significant correlation with the postoperative flexion angle. The linear regression model on the other hand revealed that preoperative flexion angle and preoperative tibiofemoral angle were the true predictors of the postoperative flexion angle with coefficients of 0.64 and -0.21, respectively. Conclusion Better range of motion before the surgery with a lower tibiofemoral varus/valgus angle were more likely to result in a better range of motion after the surgery, suggesting that an appropriate timing for the surgery when the knee joint is still in a better function can lead to a better outcome.