Our aim is to determine the respective role of calcium (Ca) and of oxalate (ox) in the formation of calcium oxalate (Caox) crystals. Two urine samples are mixed after increasing either the calcium concentration (Ca) in one sample or the oxalate concentration (ox) in the other. The crystals formed are identified by their habits or by x-ray. Similarly, urine from 37 hypercalciuric stone-formers is analysed. Caox crystallizes at an oversaturation beta greater than or equal to 13,5, critical beta. An almost linear relationship exists between beta and (ox), the critical beta being obtained at an (ox) approximately 0,26 mmol/l. No simple relationship links beta and (Ca). These results point out the predominant role of oxalate in the formation of Caox crystals.