It has been suggested that incorporation of shards of fibrillated cartilage into the synovium is a cause of synovitis in osteoarthritis (OA). We examined the prevalence with which fragments of cartilage are seen in synovium, and their association with synovitis, in patients with endstage OA and early OA of the knee. Samples of synovium were obtained from 12 patients with endstage OA who were undergoing knee joint replacement and 30 with only mild/moderate radiographic changes of OA who exhibited articular cartilage changes of OA at arthroscopy. The presence of cartilage shards was sought in synovium from the medial and lateral tibiofemoral compartments and the suprapatellar pouch of each patient. Comparable volumes of the synovial lining from patients with endstage and early OA were examined, and tissue mononuclear cell infiltration was graded as an indicator of synovitis. Cartilage shards were seen in synovium from 7 of 12 patients with endstage OA, all of whom had synovitis. No topographic relationship was found between shards and mononuclear cell infiltration. In contrast, cartilage fragments were not seen in synovium from any of the 30 patients with early OA, although 9 of them had full thickness cartilage ulcers and 17 had evidence of synovitis.