56 patients with medial compartmental osteoarthritis and genu varum were randomized to undergo a high tibial osteotomy with or without an intra-articular injection of cultured autologous bone marrow-derived mesenchymal stem cells (MSC), administered 3 weeks post-treatment. The purpose of this study was to evaluate the clinical outcomes and MRI results of both the cell-recipient and control groups to determine whether bone marrow-derived MSC injections are safe and effective. Results after 2 years indicated that the cell-recipient group had significantly improved clinical outcomes (Tegner, Lysholm and International Knee Documentation Committee scores) and MRI results (cartilage coverage of their lesions and integration of regenerated cartilage) compared to the control group. There were no serious adverse events reported in either group.