The ultrasound records of 30 fetuses suspected of having an obstructive uropathy were reviewed retrospectively. A prenatal karyotype was obtained with amniocentesis on each patient. After delivery, neonatal urologic records, renal ultrasound reports and autopsy information were reviewed and compared to the ultrasound records and fetal karyotype results. Chromosomal defects were found in 23% of fetuses with a suspected obstructive fetal uropathy. In five patients the chromosomal abnormality was lethal and caused 45% of the perinatal deaths in this series. If a fetus with an obstructive uropathy was female, there was a significant likelihood of an extrarenal anomaly or a complex genitourinary tract malformation. Seventeen percent of patients with an obstructive uropathy had a coexistent extrarenal defect. A prenatal karyotype should be obtained if a fetal obstructive uropathy is suspected antenatally since lethal chromosomal defects are an important cause of perinatal death. A female karyotype may indicate a fetus at higher risk of extrarenal anomalies or complex genitourinary malformations.