Objective: Several studies stated on medial open wedge tibial osteotomy but there is still some debate about the acceptable amount of pre-operative flexion contracture degree. Also, clinical effects of alteration of tibial slope after the procedure is not clear. This study aimed to investigate the mid-term clinical and radiological findings and complications of medial open wedge tibial osteotomy. Materials and methods: 44 knees of 42 patients were investigated retrospectively between January 2001 and February 2012. Tibial sagittal slope, mechanical tibiofemoral angle (mTFA), mechanical lateral distal femoral angle (mLDFA) and Medial Proximal Tibia Angle (MPTA) were measured, pre and post-operatively. Clinical outcome was evaluated with HSS (Hospital for Special Surgery) Knee Score, OKS (Oxford Knee Score) and KOSADLS (Knee Outcome Survey-Activities of Daily Living Scale). Results: The mean age of the participants was 45.7 ± 18.3 (range, 17-84), and the mean follow-up was 92 ± 7 (range, 70-113) months. Gender distrubition was 34 (%81) female and 8 (%19) male. 10 degrees of flexion contracture was present in 4 (%10) patients preoperatively. Mean knee range of motion was increased from 120 ± 11 to 130 ± 9 degrees, postoperatively. HSS scores were improved to excellent in 29 (69%), good in 9 (21%) and moderate in 4 (10%). ADLS and Oxford scores were improved two-fold. Conclusion: Ten degrees of flexion contracture may not be a restraint for osteotomy, with the use of technical details to prevent slope increase. Even with the rise of the slope, up to 10 degrees of flexion contracture could be corrected without anterior wedge resection or posterior capsular release. Level of evidence: Level IV, Case series, case control study (diagnostic studies), poor reference standard, analyses with no sensitivity analyses.