We describe the first case of maternal uniparental disomy (UPD) of chromosome 9 in a fetus who was shown to have mosaic trisomy 9 in a chorionic villus sample. Karyotyping and molecular studies following termination of the pregnancy confirmed mosaicism in the placenta and maternal UPD(9) in the fetal tissues. This case demonstrates that the mechanism of trisomy correction may result in a fetus with UPD(9).