Eight-two consecutive fetuses with ultrasound evidence of isolated pyelectasis (defined as dilation in the antero-posterior renal pelvic dimension of > or = 4 mm) were prospectively followed to determine the risk of postnatal uropathy and Down syndrome. In 98 (60%) kidneys, isolated pyelectasis was shown to be the first manifestation of a pathophysiological process that evolved into a gamut of postnatal uropathies (defined as urological conditions requiring remedial surgery or extended medical surveillance). Data quantifying the risk for postnatal uropathy in fetuses with varying degrees of isolated pyelectasis, at different gestational ages, are presented in figure format to facilitate prenatal counselling. Bivariate analysis showed that the evolution of isolated pyelectasis to uropathy was statistically significant when in utero progression was noted or in conjunction with other findings including contralateral pyelectasis (p < 0.01), male gender (p < 0.01) and increased kidney length (p < 0.001). Importantly, 55% of the infants requiring corrective surgery demonstrated in utero progression of pyelectasis (p < 0.002). Serial ultrasound examinations were necessary to evaluate progression or regression in the extent of pyelectasis. Finally, isolated pyelectasis was associated with an increase risk for Down syndrome, beginning at maternal age of 31 years, in the interval of 16-20 week's gestation.