The kidneys of 20 asymptomatic pregnant women were examined sequentially from the first trimester through the postpartum period. In 41.5 per cent, progressive dilatation, more marked on the right side, was seen; however, the remainder demonstrated varying, static, or even diminishing ectasia. In all but two dilatation had reverted to normal by six weeks post partum. The finding of minimal dilatation speaks against pathologic obstruction even in the symptomatic patient. Asymptomatic patients can, however, show variable degrees of hydronephrosis with progression or regression through the course of gestation. The finding of dilatation must be interpreted with caution by virtue of these sequential variations.