Apatite crystals can be associated with erosive arthritis. This report describes 3 such patients, 2 of whom had idiopathic apatite crystal deposition and 1 who had apatite related to renal failure and chronic dialysis. Diagnosis was delayed for several years in 1 patient. Roentgenograms showed calcifications in all patients. Synovial fluid had non-birefringent chunks and globules suggestive of apatite. Diagnosis was established by electron microscopy in all 3 patients with verified by x-ray diffraction in 2.