The enzyme nucleoside triphosphate pyrophosphohydrolase (NTPPPH) is present in all joint fluids and on intraarticular cells. It generates inorganic pyrophosphate (PPi) from nucleoside triphosphate substrate, thus serving as a potential source of the PPi which forms in the cartilages of patients with calcium pyrophosphate dihydrate (CPPD) crystal deposition. NTPPPH is also important in matrix vesicle induced calcification with basic calcium phosphate (BCP) crystals. An articular substrate for this enzyme was sought. ATP was measured in the joint fluids from 107 patients with various forms of arthritis. Synovial fluid ATP levels were higher in patients with CPPD deposits than in osteoarthritis (p less than 0.02) or rheumatoid arthritis (p less than 0.002). ATP also correlated with PPi concentration (p less than 0.05) and with the presence of BCP crystals (p less than 0.05), but not with cellularity of the fluid, NTPPPH activity, or age of the donor. This substrate for NTPPPH may contribute to CPPD crystal deposition by generating PPi and may stimulate matrix vesicle induced formation of BCP crystals in several forms of arthritis.