Objective: To determine the incidence of aneuploidy among fetuses and infants conceived through intracytoplasmic sperm injection (ICSI) in our clinic using umbilical cord blood samples. Design: Follow-up study of the cytogenetic outcome of ICSI pregnancies. Setting: University-based IVF clinic. Patient(s): Forty-six couples who underwent ICSI and conceived. Intervention(s): Umbilical cord blood was taken after delivery of the infant for analysis. Samples of chorionic villi and chorion were taken for studies on the spontaneous abortuses. Amniocentesis was performed for couples that chose prenatal diagnosis. Main Outcome Measure(s): The cytogenetic chromosomal status of the pregnancy outcome. Result(s): Fifty pregnancies and 55 live births were recorded, with nine spontaneous abortions. Of 43 separate umbilical cord blood samples analyzed, I abnormality (2%) was found, 45, XX, +21. Nine births went through prenatal diagnosis alone. with four accepting both forms of analysis-no abnormalities were found. Origin of abnormality was established in two spontaneous abortion cases (45, XO and 45, XY,-21). and the maternal chromosome was lost in both cases. Conclusion(s): Using umbilical cord blood obtained after birth, we obtained karyotype results from 78% of the ICSI population in our clinic. Combined with results from five additional cases that underwent prenatal diagnosis but not umbilical cord blood sampling, a chromosomal result was obtained in 87% of our ICSI population. The use of umbilical cord blood for cytogenetic analysis substantially improves the ability to determine rates of chromosomal abnormalities in newborns produced via ICSI clinics. (C) 2001 by American Society for Reproductive Medicine.