To evaluate routine fetal genitourinary tract obstetrical ultrasound screening, and to determine what size renal pelvis is indicative of significant renal disease. Examinations (4832), which were performed over 2 yr, or 3530 consecutive obstetrical patients. Any fetus that had a renal pelvis > 5 mm or a definable cystic area was identified for follow-up. The fetuses of 39 patients (1.1%) who underwent 112 examinations fulfilled these criteria and constitute the basis of this report. A variety of examination criteria were recorded and analyzed in relationship to the follow-up, which ranged from 2-3 days to 21 mo. The fetuses of the 39 patients were grouped into 3 categories: those with renal pelves between 5 and 9 mm in size; those with renal pelves > 10 mm; and those with cystic abnormalities. Those with renal pelves > 10 mm had either an obstructing lesion or exceptional extrarenal pelves. The clinical and pathologic aspects of these 3 groups are detailed, discussed and analyzed. Criteria for significant fetal renal hydroenphrosis and aspects of a loculated appearance are given.