In 7 highly recurrent calcium oxalate stone-formers and 10 healthy subjects the effects of urine on three processes of calcium oxalate monohydrate crystallisation--solubility, crystal growth, and crystal agglomeration--were studied. The urine of the stone-formers showed low calcium oxalate solubility and normal crystal growth inhibition, but lacked the ability to inhibit crystal agglomeration. As the sole metabolic abnormality, all stone-formers showed hypocitraturia. Normalisation of urinary citrate concentration resulted both in vitro and in vivo in a significant rise in agglomeration inhibition. These results show that inhibition of agglomeration is a very important, probably citrate-regulated, process in calcium oxalate stone formation.