To show that the position of the choroid plexus is dependent on gravity and to prove that this fact can be used as a simple means of avoiding the erroneous diagnosis of ventriculomegaly on fetal sonography, we evaluated 75 fetal sonograms retrospectively. Twenty-five fetuses had ventriculomegaly, and 50 had normal cerebral ventricles. The gestational ages ranged from 15 to 39 weeks. To show objectively that the position of the choroid plexus within the lateral ventricle was gravity dependent, we measured the choroid angle in each case. The choroid angle (the angle between the long axis of the choroid plexus and the linear midline echo on transverse axial sonograms through the body of the lateral ventricles) varied directly with ventricular size. In the group with normal-sized ventricles, the values for choroid angle followed a normal, unimodal distribution and had a mean of 14 degrees, a range of 6-22 degrees, and an SD of 4.3 degrees. In cases of ventriculomegaly, the values for choroid angle did not follow a normal distribution and ranged from 29 to 90 degrees. The choroid "dangled" from its attachment at the foramen of Monro and rested on the dependent wall of the lateral ventricle, resulting in a choroid angle that was increased over normal; the degree of the angle was dependent on the severity of the ventricular enlargement. The resting position of the choroid plexus marked the position of the lateral ventricular wall even when the reflection of ultrasound from the ventricular wall itself could not be seen. Detection of the position of the dependent choroid plexus is a simple observation that can be used to avoid the erroneous diagnosis of fetal ventriculomegaly and to help gauge the severity of true ventricular enlargement.