Primary congenital megalo-ureter is the occurrence of enormous bilateral caliber of the ureter in cases in which there is no demonstrable obstruction to the urinary outflow as determined at necropsy or by complete examination during life. The condition has been observed in the fetus, in the new-born and in early life. Similar cases to those reported here appear in the literature under the title of congenital dilatation of the ureters. The term dilatation is ill chosen because the so-called dilated ureters of the type to be described here probably never were of normal caliber and may represent the persistence of a stage in early fetal development. The expression dilatation of the ureters should be reserved for those cases in which there is definite pathologic condition or anomaly that accounts for the widening of the ureteral lumen. These cases may be congenital or acquired. In fact, the condition may exist in