The effects of an orally administered combination of a glucosamine-chondroitin-quercetin glucoside (GCQG) supplement on the synovial fluid properties of patients with osteoarthritis (OA) and rheumatoid arthritis (RA) were investigated from the clinical nutrition view point. In this study, forty-six OA and twenty-two RA patients were administered with the GCQG supplement orally for 3 months. Several parameters of the knee joints were monitored before and after supplementation. The OA patients showed a significant improvement in pain symptoms, daily activities (walking and climbing up and down stairs), and visual analogue scale, and changes in the synovial fluid properties with respect to the protein concentration, molecular size of hyaluronic acid, and chondroitin 6-sulphate concentration were also observed. However, no such effects were observed in the RA patients. These results suggest that the GCQG supplement exerted a special effect on improving the synovial fluid properties in OA patients.