1. The degree of saturation with calcium oxalate has been determined in fresh urine samples from six patients with recurrent calcium oxalate-containing renal stones and six normal control subjects who were studied under the same conditions of diet and fluid intake.

2. The degree of saturation of urine with calcium oxalate was significantly higher in the group of stone-formers than in the control series and more often exceeded the amount needed for spontaneous crystallization of calcium oxalate (formation product). This was accounted for by the higher concentration of calcium and oxalate in the urine of the stone-formers.

3. Crystals of calcium oxalate were observed in all freshly examined urines in which the formation product of calcium oxalate was exceeded. Since the formation product of calcium oxalate was exceeded more often in the urines of stone-formers than in the urines of the control subjects, this accounted for the greater calcium oxalate crystalluria of the stone-formers.

4. Addition of a small quantity of sodium oxalate to the basal diets of the two groups resulted in a greater increase in the urine saturation and calcium oxalate crystalluria of the stone-formers, thus accentuating the difference observed between the two groups when they were given the basal diet.

5. Calcium oxalate crystalluria was related quantitatively to the degree of over-saturation of urine with calcium oxalate, although uric acid solubility may play a small role at low pH values.

6. The results are consistent with a ‘hyperexcretion—crystallization’ mechanism of stone formation.

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