Synovial fluid from eighty patients (eighty-eight joints; eighty-seven knees and one ankle) was studied by polarized light microscopy for crystal detection and identification. These patients were also studied by plain roentgenography and by arthroscopy. Crystals were identified in the synovial fluids of forty-two joints (forty-one knees and one ankle). Arthroscopy revealed crystalline deposits in seventeen joints (sixteen knees and one ankle), and sixteen of the seventeen joints had a dramatic decrease in pain following joint perfusion. Those patients with no evidence of crystalline deposition in the roentgenograms or at arthroscopy, but who had crystals identified in their synovial fluid, occasionally had dramatic relief of pain. Generally moderate but not universal relief was seen in this group. Several patients in a control group were markedly improved following arthroscopy. These were patients with extensive degenerative changes in their hyaline cartilage and tears of their fibrocartilage. It was thought that these patients were continually seeding their joints with microscopic and macroscopic cartilaginous debris.