The formation of renal calculus is, according to the currently accepted view, a polyetiological process whose most important components are the composition of the urine and urinary stasis. The second of these factors is discussed. The importance of anatomical lesions is indicated by the finding of a hindrance of flow in 66 of the 100 patients operated for renal calculus. The stasis may, however, have some functional cause, too. X-rays revealed two types of hypomotility: a temporary, or in certain parts constant, lack of contractions and sluggish contractions of normal frequency. Temporary lack or sluggishness of contraction promotes calculus formation because the physiological pyelo-calicous reflux which occurs in three quarters of the intact cases is lacking. These refluxes counteract sedimentation and together with the orthostatic emptying are the most effective factors preventing the retention of calculus nuclei.