Apatite crystals, calcium pyrophosphate dihydrate crystals, or both were observed in knee joint effusions from 60% of 100 consecutive osteoarthritis (OA) patients. Crystals were mor common in patients with more severe OA and in joints that had received previous intraarticular steroid injections. Whether the latter was predominantly related to an effect of the steroid or to the fact that steroids were used in more severe cases was not clear. Crystals existed in many effusions without elevated synovial fluid leukocyte counts. A subgroup of 7 patients had both types of crystals, more crystal phagocytosis, and more severe OA.