Increased catalytic activity of synovial-type (group II) phospholipase A2 (syn-PLA2), has been associated with cartilage erosions in rheumatoid arthritis and osteoarthritis. The catalytic activity of phospholipase A2 and the concentration of syn-PLA2 were measured in a prospective study in synovial fluid (SF) samples from 66 patients with acute knee joint effusion. The median (range) of the concentration of syn-PLA2 in SF was 210 μg/1 (80-1480 μg/1) in culture-positive septic arthritis, 460 μg/1 (270-1040 μg/1) in reactive arthritis, 780 μg/1 (120-2710 μg/1) in osteoarthritis and 230 μg/l (HO-1400 μg/1) in traumatic joint effusions. High concentrations of syn-PLA2 are found also in SF of patients with arthritides not expected to lead to permanent destruction of cartilage.