1. The volume, size and type of calcium oxalate crystals excreted in the urine of a group of patients with recurrent ‘idiopathic’ stones were studied on a controlled basal diet, after an oral supplement of sodium oxalate and after oral administration of ethane-1-hydroxy-1,1-diphosphonate (EHDP) for 4 weeks.

2. Before administration of EHDP the stone-formers passed the large crystals and aggregates of calcium oxalate dihydrate characteristic of recurrent calcium oxalate stone-formers. For the same level of urine saturation and crystalluria EHDP caused a significant reduction in the proportion of large crystals and aggregates excreted. Studies by light-microscopy confirmed that EHDP caused a striking change in the size and habit of calcium oxalate crystals in some but not all of the urine samples examined.

3. The decrease in average crystal size during the administration of EHDP was attributed to the observed increase in the ability of urine to inhibit the growth and aggregation of calcium oxalate crystals as measured by a growth system in vitro.

4. The possible use of EHDP as a therapeutic agent in the treatment of calcium oxalate stone-formation is discussed.

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