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 micrograms/l (80-1480 micrograms/l) in culture-positive septic arthritis, 460 micrograms/l (270-1040 micrograms/l) in reactive arthritis, 780 micrograms/l (120-2710 micrograms/l) in osteoarthritis and 230 micrograms/l (80-1400 micrograms/l) 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.