Nine patients developed an acute, aseptic, periarthritic inflammation associated with soft tissue calcification during treatment with maintenance hemodialysis. Neither crystals of uric acid nor calcium pyrophosphate dihydrate were identified in the joint or tissue fluid. Probably, calcium hydroxyapatite deposits were responsible for the inflammation. The acute attacks were treated with phenylbutazone with generally good clinical response. Recurrent attacks of periarthritis can often be prevented by maintaining normal levels of serum phosphorus with sufficient quantities of phosphate-binding antiacids and greater frequency of adequate dialysis.