In order to evaluate the prognostic significance of ultrasonographically detected fetal pyelectasis, a retrospective review was performed of patients evaluated for fetal pyelectasis over a 24-month period. From 84 patients a total of 98 fetal kidneys with pyelectasis were identified. Fetal pyelectasis was more common in the left kidney and in males. Postpartum evaluation revealed 48 (57.1%) neonates with hydronephrosis. Among these, mean (SD; range) values of antenatal fetal pyelectasis were 17.0 mm (8.4; 5-26) before 33 weeks and 16.9 mm (8.5; 5-34) after 33 weeks in the left kidney and 15.4 mm (3.3; 10-24) and 17.1 mm (5.1; 5-36), respectively, in the right kidney. Thirteen infants (15.4%) with hydronephrosis required surgical pyeloplasties (mean age 6 months; range 3-18 months). It was found, from a receiver-operating characteristic curve, that fetal pyelectasis of 8 mm was 91% sensitive and 72% specific in predicting subsequent hydronephrosis. Use of a threshold of 5 mm yielded a sensitivity of 100% and a specificity of 24%. On the basis of these findings, we recommend that women with ultrasonographically detected antenatal fetal pyelectasis of > or = 5 mm at any gestational age have follow-up ultrasound examinations and detailed postnatal evaluation.