The presence of multiple ultrasonographic abnormalities is associated with a significantly increased risk of chromosomal defects, while for isolated abnormalities, the association is less clear. In a study of 1,177 fetuses with mild hydronephrosis at 16-26 weeks of gestation, the fetal karyotype was abnormal in 86 (7.3%) of the cases and the most common chromosomal defects were trisomies 21, 18 and 13. The frequency of chromosomal defects increased with the number of additional abnormalities and for each chromosomal defect there was a characteristic pattern of associated abnormalities. However, in the 805 fetuses with apparently isolated hydronephrosis there were 5 (0.62%) with trisomy 21. On the basis of the maternal age and gestational age distribution of the population the expected frequency of trisomy 21 was 0.40%, which was not significantly different from the observed (0.62%). To demonstrate that such a difference is significant, it would be necessary to investigate at least 1 million pregnancies. In the meantime, parents could be counselled that the presence of mild hydronephrosis does not increase significantly the risk that the fetus has trisomy 21. Alternatively, the risk is 1.6 times higher than the maternal age and gestational age-related risk.