Needle-shaped crystals of 75 to 250 A diameter have been identified by transmission electron microscopy in clumps within synovial fluid mononuclear cell vacuoles in a variety of joint diseases. These crystals, similar to those previously associated with calcific periarthritis, were seen in acute undiagnosed arthritis and in exacerbations of osteoarthritis where they may be inducing a synovitis similar to that seen with urate and pyrophosphate crystals in gout and pseudogout. By light microscopy purple staining cytoplasmic inclusions or extracellular globules can suggest the presence of clumps of these crystals. Apatite clumps can also occasionally appear as small birefringent chunks or rods and thus might mimic urate or calcium pyrophosphate. Ultrastructural appearance, electron probe analysis, and X-ray diffraction pattern were those of apatite. Experimental injection of hydroxyapatite crystals into dog knee joints produces inflammation supporting the potential role for these crystals in joint disease.