Restoring knee stability through reconstruction, while providing symptomatic relief, has not been shown to decrease the incidence of degenerative changes after rupture of the anterior cruciate ligament. This suggests that posttraumatic osteoarthritis may not be purely biomechanical in origin, but also biochemical. To test this, we measured the levels of seven cytokine modulators of cartilage metabolism in knee joint synovial fluid after anterior cruciate ligament rupture. We also measured keratan sulfate, a product of articular cartilage catabolism. The sample population consisted of patients with uninjured knee joints (N = 10), and patients with acute (N = 60), subacute (N = 18), and chronic (N = 8) anterior cruciate ligament-deficient knees. Synovial fluid samples were analyzed by enzyme-linked immunosorbent assays. Normal synovial fluids contained high levels of the interleukin-1 receptor antagonist but low concentrations of other cytokines. Immediately after ligament rupture there were large increases in interleukins 6 and 8, tumor necrosis factor alpha, and keratan sulfate. Interleukin-1 levels remained low throughout the course. As the injury became subacute and then chronic, interleukin-6, tumor necrosis factor-alpha, and keratan sulfate levels fell but remained considerably elevated 3 months after injury. Concentrations of interleukin-1Ra fell dramatically. Granulocyte-macrophage colony-stimulating factor concentrations were normal acutely and subacutely but by 3 months after injury were elevated 10-fold. Our data reveal a persistent and evolving disturbance in cytokine and keratan sulfate profiles within the anterior cruciate ligament-deficient knee, suggesting an important biochemical dimension to the development of osteoarthritis there.