Background: Bone marrow aspiration (BMA) is among a group of autologous cell-based therapies currently being explored to treat osteoarthritis (OA). Materials & methods: This retrospective case study observed ten patients (13 knees) with severe knee OA who failed extensive conservative treatment and were treated with BMA injection using a novel, pure bone marrow aspiration (pBMA) technique. Results: No adverse events were reported. More than 50% reduction in the visual analog scale score for pain was observed at 2- and 12-weeks post-procedure, showing statistical significance. At 64 ± 26 weeks post-procedure, average knee pain remained significantly less than pre-procedure. Conclusion: Therefore, patients with severe knee OA may achieve significant relief after a BMA injection obtained using this pure bone marrow aspiration technique.