Objective: To evaluate lymphocyte populations in stifle synovium and synovial fluid of dogs with degenerative cranial cruciate ligament rupture (CCLR). Study Design: Prospective clinical study. Animals: Dogs (n=25) with stifle arthritis and CCLR, 7 dogs with arthritis associated with cartilage degeneration (osteoarthritis [OA]), and 12 healthy Beagle dogs with intact CCL. Methods: Arthritis was graded radiographically in CCLR dogs. After collection of joint tissues, mononuclear cells were isolated and subsequently analyzed using flow cytometry for expression of CD3, CD4, CD8, and CD21. Results: The proportions of CD4 + T helper lymphocytes, CD8 + cytotoxic T lymphocytes, and CD3 +CD4 -CD8 - T lymphocytes were increased in synovium from dogs with CCLR compared with synovium from healthy Beagle dogs (P<.05). The proportion of CD3 +CD4 -CD8 - T lymphocytes in synovial fluid was increased in dogs with CCLR compared with dogs with OA (P<.05). In dogs with CCLR, the proportion of CD3 +CD4 -CD8 - T lymphocytes in synovial fluid was inversely correlated with radiographic arthritis (S R=-0.68, P<.005). Conclusion: Lymphocytic inflammation of stifle synovium and synovial fluid is an important feature of the CCLR arthropathy. Lymphocyte populations include T lymphocytes expressing CD4 and CD8, and CD3 +CD4 -CD8 - T lymphocytes. Presence of CD3 +CD4 -CD8 - T lymphocytes was associated with development of stifle synovitis. Further work is needed to fully identify the phenotype of these cells.