Ewing's sarcoma presenting as monoarthritis.

A 22-year-old woman presented with a monoarthritis which mimicked septic arthritis. Synovial biopsy showed chronic synovitis. The synovial fluid was macroscopically purulent but was subsequently shown to contain malignant cells arising from a Ewing's sarcoma of the distal femur. This case indicates that malignancy should be considered in the differential diagnosis of an unexplained monoarthritis, and documents a monoarthritis as the presenting feature of Ewing's sarcoma.