We measured levels of cartilage proteoglycan (PG) fragments in knee joint synovial fluid obtained from patients with previous trauma of the knee, early gonarthrosis, or pyrophosphate synovitis, and in age-matched control subjects. During the initial 3–4 weeks after rupture of the anterior cruciate ligament or the meniscus (confirmed by arthroscopy), markedly increased PG fragment levels were found. At later times after trauma (up to 4 years), many of these patients still had significantly elevated levels of cartilage PG fragments in the joint fluid. In a group of older patients with gonarthrosis, these levels were only moderately elevated, while in patients with acute pseudogout, greatly increased levels were observed. Although longitudinal studies are needed to validate the significance, PG fragments in joint fluid may be a marker for early posttraumatic arthrosis.