OBJECTIVES The prognosis of borderline forms of anomalies that can be detected by ultrasound is one of the most challenging issues in prenatal diagnosis. The aim of this study was to determine the prognosis for fetuses presenting with isolated mild ventriculomegaly (MVM). METHODS Fetuses in which the width of the lateral ventricular atria was 10-12 mm and which had no other detectable chromosomal or morphological anomalies were followed by monthly ultrasound examinations until delivery. For the cases identified up to December 1997, postnatal information was gathered retrospectively through interviews. Children born from January 1998 onwards were included in a protocol involving planned neuropsychiatric visits at 12 and 18 months of age in which the Griffith scale was used to assess neurodevelopment. RESULTS Between September 1992 and January 2001, 60 fetuses with isolated MVM were identified. Ventricular dilatation diminished in 18 cases (and became normal in nine of these) and stabilized in 42 cases. Information was obtained on 38 children born up to December 1997 and their neurodevelopment was found to be completely normal. The 22 children born from January 1998 onwards showed normal development at 12 and 18 months of age. CONCLUSIONS When MVM is observed on prenatal ultrasound examination it can be very difficult to offer parents appropriate counseling. It is important to exclude aneuploidy or morphological abnormalities but even then there will be anxieties about long-term neurological outcome. Our data, which show normal neurodevelopment between 18 months and 10 years after birth in cases of MVM (10-12 mm), should provide a basis for reassuring counseling.