To investigate the relationship between classification and prognosis of hydronephrosis in fetus.226 pregnant women in their pregnant weeks 20 - 40 who were suggested to be with fetuses suffering from hydronephrosis underwent ultrasonography to observe the configuration of the kidneys, form and size of renal pelvis, extent of calyces, and thickness of renal cortex of their fetuses. The ultrasonography was conducted regularly and the outcome after birth was followed up.143 fetuses (186 kidneys) were diagnosed as with hydronephrosis of grade I with the anteroposterior diameter of the renal pelvis from 0.3 to 1.1 cm that fadeawayed soon after birth with a good prognosis. 47 fetuses (52 kidneys) were diagnosed as with hydronephrosis of grade II with the anteroposterior diameter of the renal pelvis from 1.0 to 1.8 cm, most of which remised gradually after birth, and only about 5% of which became worse along with the time of pregnancy and needed surgery after birth. 10 fetuses (10 kidneys) were diagnosed as with hydronephrosis of grade III with the anteroposterior diameter of the renal pelvis from 1.5 to 3.3 cm about 70% of which showed a tendency to deteriorate along with the time of pregnancy and after birth, and the neonates needed surgery after birth. Eight fetuses (10 kidneys) were diagnosed as with hydronephrosis of grade IV with the anteroposterior diameter of renal pelvis from 1.5 to 7.2 cm that needed surgery after birth.Follow-up and monitoring are not necessary for those fetus with hydronephrosis of grade I; however, regular ultrasonography is needed for the those with hydronephrosis of grade II and over.