The effect of serum and synovial fluid obtained from six healthy subjects and from 12 patients with gout, six with rheumatoid arthritis and 18 with calcium pyrophosphate dihydrate arthropathy (CPPD) on the rate of in vitro urate crystal formation was measured. Gouty and normal serum produced similar results. Gouty synovial fluids promoted urate crystal formation significantly more than did rheumatoid or CPPD fluids. The promotion of urate crystal formation by gouty synovial fluid was not due to native urate crystals nor to an effect on the level of urate supersaturation. The development of gout in some, but not other, hyperuricaemic subjects may relate to the presence of promoters (or relative lack of inhibitors) of urate crystal formation in the group who develop gout.