Significant recent advances in ultrasonic technology have now made possible detailed evaluation of normal and abnormal fetal intracranial anatomy. Accurate knowledge of fetal intracranial anatomy is important for accurate and reproducible cephalometry as well as in antenatal diagnosis of dilated brain ventricles. High resolution real-time ultrasonic investigation was performed in 280 normal obstetric patients from the 14th to the 41st weeks of pregnancy. Eight patients with antenatally detected hydrocephalic fetuses were also included in the study. Measurements of the ratio between lateral ventricular width and cerebral hemispheric width (LVW/HW) showed value at 0.61 of the 14th week to 0.29 at about the 27th week. This value then remains unchanged until term. Accordingly the LVW/HW ratio decreases as the fetus grows reflecting the developmental changes. Due to this simple measurement technique, it is possible to diagnose hydrocephaly much earlier than by means of the identification of an abnormal BPD growth or a discrepancy between head and abdomen circumferences. Although the earliest reported cases of prenatal ultrasound detection of hydrocephaly have been at 17-18 weeks, one must be very careful when making the diagnosis prior to 20 weeks because the lateral ventricles are usually disproportionally large at this time and the LVW/HW ratio is, therefore, high. Moreover, the normal range is rather wide at this age. For this reason it is useful to repeat the ultrasound examination after one week; if the ratio increases, the diagnosis of hydrocephaly can be made without any doubt.