In a double-blind controlled clinical study, 71 patients with recurrent calcium oxalate stones were divided into three treatment groups: those who received potassium acid phosphate, those who received an inert placebo, and those who received a low calcium diet only. Follow-up periods averaged 2.9 years. Although the mean urinary calcium level of the patients who received phosphate was reduced 33 per cent, their renal stone disease did not diminish. Mean urinary phosphorus increased 88 per cent with phosphate treatment but did not correlate with the decrease in urinary calcium, or with treatment success. The data did not suggest that phosphorus and its metabolites retard calcium oxalate crystallization in urine. No evidence appeared for an association of hypercalciuria with severe stone disease, or with a specific clinical or chemical response to phosphate therapy. Patients whose urinary calcium level fell more than 25 percent when dietary calcium was reduced may have excessive gastrointestinal calcium absorption, which appears to be associated with improved chemical response to phosphate therapy.