Urate crystals in synovial fluid and the crystals from a subcutaneous tophus were found to have identical optical properties by polarized light microscopy. They were negatively birefringent with extinction on the long axis. Urate crystals were identified by polarized light microscopy in the aspirated synovial fluid obtained from 15 to 18 patients with clinical gout. The crystals were specifically digested by uricase in all 15 samples and in two instances disappeared in the control specimens as well. Urate crystals were also identified by ordinary light microscopy in 11 of the same 18 synovial fluid samples. Comparison of the results of ordinary light microscopy with those seen by polariscopic observation revealed that the percentage of positive identification was greater when the latter method was used. In two instances, crystals were seen by ordinary light which did not have the characteristics of urate crystals. One of these was from an otherwise typical case of acute gouty arthritis. The concentration of urate in the synovial fluid during the acute attack is probably higher than that found in the serum.