OBJECTIVE: To determine the fetal aneuploidy in fetal blood cells from cordocentesis. METHODS: We analyzed their karyotype and performed fluorescence in situ hybridization(FISH) for chromosome 18, 21, X, and Y in 14 cases of fetal blood cells from cordocentesis at Department of Obstetrics & Gynecology, College of Medicine, Seoul National University and Hamchoon Women's Clinic. RESULTS: In all cases we obtained the consistent results in both methods and were able to rapidly detect aneuploidy in uncultured fetal blood cells using FISH before karyotyping with culture for 48 hr. The averages for accuracy of FISH were from 84.6 % to 93.9%. CONCLUSION: In this study we suggest that the rapid detection in uncultured fetal blood using FISH is possible and that this diagnostic method will be clinically useful when rapid result would be demanded.