1. In order to assess the relative importance of possible pathogenetic factors in the formation of calcium-containing renal stones, a group of 18 patients (12 men, six women) with active, recurrent stone disease were compared with 16 age-matched control subjects (10 men, six women) given an identical diet.

2. Fifteen (83%) of the patients showed at least one, eight (44%) showed two, and one (6%) patient showed three abnormalities that might predispose to stone formation.

3. Increased urinary calcium excretion was the most common abnormality (11 patients, 61%), particularly in the women (83%).

4. A diminished excretion of inhibitors of crystal formation of calcium phosphate was the next most common abnormality, which occurred in eight patients (44%), all of whom were men. It was largely attributable to a diminished excretion of inorganic pyrophosphate (PPi). The PPi/orthophosphate ratios were also lower in the stone-formers. Significant differences in residual inhibitory activity after enzymatic removal of PPi between control subjects and stone-formers could not be found in 24 h urine samples but were present during certain times of the day. Pyrophosphate showed a higher inhibitory activity in urine than in control solutions, this enhancement being absent in stone-formers.

5. Nine (50%) of the patients, but only one of the control subjects, produced crystal aggregates greater than 50 μm in diameter after an oral load of oxalate.

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