Thirty-one calcium stone patients were treated with 300 mg allopurinol daily during a mean follow-up period of 2 years. They were also given advice on diet and fluid intake. The effects on the recurrence rate of stones were compared with the effects on the recurrence rate in 43 calcium stone patients who were given similar advice on diet and fluid intake but were not treated with allopurinol. In both groups there was a reduction in the recurrence rate but no difference between patients treated with allopurinol and the control group. Some patients with only hyperuricosuria prior to treatment seemed to benefit from allopurinol treatment, whereas those with hypercalciuria continued to form stones with the same frequency. Allopurinol treatment in calcium stone disease appears less effective than treatment with thiazides, magnesium hydroxide or orthophosphate.