Ultrasound examinations during pregnancy are performed partly as a routine and partly on specific indications. Nineteen fetuses with intrauterine hydronephrosis were diagnosed during a 4-year period; 15 were inborns at the Department of Obstetrics in Lund and 4 were referred from other hospitals. Three cases were diagnosed at the first routine screening in the 17th week of gestation and the remaining 16 at a later screening examination at 32 – 36 weeks gestation. The size of the renal pelvis was measured at two perpendicular diameters in a transverse scan of the fetal abdomen. All infants were examined during the 1st week postpartum and re-examined within the 1st month of life with ultrasound and intravenous urography. Increasing dilatation of the renal pelvis was an indication for operation. There were no false-positive cases. A significant correlation between the measured mean diameter of the renal pelvis and postnatal treatment was found. When the antenatal pelvic diameter is more than 10 mm careful postpartum controls are recommended. All kidneys with a pelvic diameter of more than 20 mm were operated upon.