Osteoarthritis (OA) is a disease that primarily results in the degeneration and destruction of the articular cartilage. However, synovitis that occurs secondarily by this primary phenomenon is crucial for both the structural and symptomatic progression of the disease. The Japanese Knee Osteoarthritis Measure (JKOM) was created as an outcome measure for Japanese patients with knee OA. This study was conducted to determine whether synovitis in knee OA correlates with the current disability of patients with knee OA who required total knee arthroplasty (TKA). Thirty-four Japanese patients with end-stage knee OA who required TKA were included in this study. The visual analog scale (VAS, 0-100) for pain and the JKOM score, as well as the Western Ontario and McMaster Universities Arthritis Index (WOMAC), were examined before the operation. Synovial samples were taken at the time of the operation. A histological analysis and gadolinium-enhanced magnetic resonance imaging (Gd-MRI) were conducted to evaluate synovitis. Correlations between the synovitis score evaluated by histological analysis and Gd-MRI with either the pain VAS score or the JKOM score were examined using Spearman's rank correlation coefficient. Neither the synovitis scores evaluated by the histological analysis nor those by a Gd-MRI correlated with the pain VAS score (n = 34, r = 0.25, p = 0.18 and r = 0.08, p = 0.75, respectively) and WOMAC (n = 14, r = 0.35, p = 0.22 and r = 0.45, p = 0.16, respectively) of the patients. However, they significantly correlated with the JKOM score of the patients (n = 34, r = 0.55, p = 0.001 and r = 0.71, p = 0.001, respectively). The severity of synovitis in OA was closely correlated with the current functional impairment and disability of the patients receiving TKA with end-stage knee OA.