The occurrence of hydronephrosis and hydroureters during pregnancy has been termed physiological, inasmuch as it is seen in more than 80%, most frequently and most pronounced in primigravida. The dilatation develops during the second trimester, and becomes more prominent on the right side, is only seen above the linea terminalis and disappears within a few weeks after birth. The condition is not seen in women whose ureters do not cross the pelvic brim. The extent of the dilatation can be reduced by placing the woman on the side least affected or in the knee-elbow position. After a survey of the literature, the conclusion is reached that today there is every probability that hydronephrosis during pregnancy develops as a result of compression of the ureters between the pregnant uterus and the linea terminalis. It has not been demonstrated that the change in hormonal balance during pregnancy is of importance. The clinical significance of hydronephrosis lies in the association between ureteral obstruction and the high frequency of ascending urinary tract infection during pregnancy and in the understanding of the importance--in the treatment of infection--of improving drainage by means of a change in position. Isotope renographic studies seem to show a possible relationship between bilateral ureteral obstruction and the development of pre-eclampsia. Similarly, it appears that acute hydronephrosis or worsening of an existing hydronephrosis has been somewhat overlooked as a possible cause of uncertain abdominal pain during pregnancy. These conditions should be examined by means of ultrasonography, and an attempt at treatment by a change in position should be made. In cases of continued pain or affected renal function, treatment should consist of the insertion of a ureteral catheter.