Synovial fluids from 2 of 2 patients with osteoarthritis, 2 of 8 with rheumatoid arthritis and 2 of 2 with ankylosing spondylitis contained a low MW angiogenesis factor apparently identical with that derived from tumors. The amount of factor present was highest in patients with osteoarthritis. Two patients with psoriatic arthropathy gave negative results. Angiogenesis factor may be involved, directly or indirectly, in the production of synovial exudate.