The pathology of failure in currently used biological treatments (cell therapies and microfracture) for cartilage repair or early osteoarthritis (OA) is poorly understood. We aimed to identify a reliable panel of biological predictors, present in synovial fluid, which can be used in the preoperative setting to optimise patient selection and therapy efficacy. The-long term aim is to reduce the number of patients who progress to end-stage OA and require knee replacement by making earlier, less invasive treatments more effective. In this first stage, we have taken synovial fluid aspirates from patients undergoing autologous chondrocyte implantation (ACI) at our centre in Oswestry, UK. Synovial fluids (SFs) were obtained from 14 ACI responders (mean Lysholm improvement of 33 (17-54)) and 13 non-responders (mean Lysholm decrease of 14 (4-46)) at the two stages of surgery (cartilage harvest and chondrocyte implantation). Dynamic range compression of synovial fluids coupled with label-free quantification mass spectrometry (MS), was used in an unbiased approach to identify predictive markers of ACI success or failure and to investigate the biological pathways involved in the clinical response to ACI.