Design To assess the neuropsychological outcome of children with asymmetric ventricles and unilateral ventriculomegaly identified in utero. Setting Fetal neurology clinic. Population We assessed 21 children with asymmetric ventricles (group 1) and 20 children with unilateral ventriculomegaly (group 2) identified in utero and compared them with a group of 20 children with symmetric ventricles using a formal neuropsychological tool: the Bayley Scale of Infant Development II (BSID-II). Main outcome measures The group of children with unilateral ventriculomegaly scored significantly lower than the control group on the mental developmental index (MDI) and on the behaviour rating scale (BRS) but not on the psychomotor index. The group of children with asymmetric ventricles did not differ significantly from the control group on either the MDI or psychomotor developmental index but differed from the latter on the BRS. Fifteen percent of the children in the asymmetric ventriculomegaly group performed two SDs below average compared with 4% of children in the asymmetrical ventricles group and none of the control. Conclusion Our results indicate that prenatally observed unilateral ventriculomegaly is a significant risk factor for developmental delay. The mental and motor outcome of children with asymmetric ventricles is similar to that of the control group, but these children are at a significant risk for behavioural abnormalities.