Ultrasound of a fetus at 17 weeks gestation revealed posterior urethral valve syndrome with anhydramnios. Fluorescence in situ hybridization (FISH) to detect aneuploidies of chromosomes 13, 18, 21, X and Y was performed on transitional cells from the fetal bladder obtained at percutaneous vesicocentesis, followed by conventional cytogenetics. Fetal urine was chosen due to unavailability of amniotic fluid for karyotypic analysis. A nonlethal (disomic) karyotype was suggested by FISH, and thus placement of a vesicoamniotic shunt was performed. The ability to prognosticate in cases of obstructive uropathy is not absolute, and fetal surgery for relief of urinary obstruction is best performed at the earliest possible gestational age. Thus, all available means for rapidly ruling out lethal congenital anomalies should be undertaken in cases of obstructive uropathy prior to any decision regarding fetal surgery.