A study of the literature has revealed 11 cases of calcium "milk" in hydronephroses. We have been able to observe five further cases in a period of 14 months. The criteria annunciated by Murray are stressed, since they will raise a suspicion of the presence of calcium "milk" on views taken with a vertical x-ray beam. In three patients, the cause of the hydronephrosis was a stone in the pelvi-ureteric junction. In the other two cases, there was a stenosis below the pelvis. Murray's criteria permitting identification of calcium "milk" with a vertical beam were analysed. Not all his criteria were found to be reliable.