Abnormalities of the fetal urinary tract, most commonly hydronephrosis of differing causes, can be detected by ultrasound (US). Prenatal measurements of the kidney can help to predict the postnatal outcome of hydronephrosis. About 20% of abnormalities evident in utero are not present after birth. After delivery, various imaging procedures, particularly US and voiding cystourethrography may be necessary to evaluate prenatally diagnosed abnormalities. Pyelectasis may be due to obstruction and this may be assessed by sonography during diuresis, urography or diuretic renography. Renography can also evaluate global and split function of the kidneys. 123I-hippurate and 99 mTc-MAG3 are superior to 99 mTc-DTPA, but there is no 'gold standard' technique available to assess obstruction. The majority of cases of hydronephrosis, even those which appear to be due to obstruction, can be treated conservatively without deterioration of renal function. Primary megaureter is the second most common cause of obstructive hydronephrosis in the newborn. The diagnostic procedures are similar if the dilated ureter persists unchanged and the infant is asymptomatic. Newborns with antenatally detected renal abnormalities frequently have associated vesicoureteric reflux.