In 267 consecutive cases of fetal lateral cerebral ventriculomegaly, additional fetal malformations were detected by ultrasonography in 209 (78%) of the cases. On the basis of the ultrasound findings, the patients were subdivided into three groups: (i) isolated ventriculomegaly (n = 58), (ii) ventriculomegaly and open spina bifida only (n = 172), and (iii) ventriculomegaly and other malformations (n = 37) with or without spina bifida. Antenatal karyotyping was performed in 64 cases from groups (i) and (iii), and 11 (18%) of the fetuses had chromosomal abnormalities. The incidence of chromosomal abnormalities was strongly related to the presence of multisystem malformations. Thus, only 3% of fetuses with isolated ventriculomegaly as opposed to 36% of those with additional malformations had chromosomal defects. Furthermore, the degree of ventriculomegaly in the chromosomally abnormal fetuses was relatively mild. In the chromosomally normal fetuses, mild, static ventriculomegaly was associated with apparently normal subsequent mental development.