The ventricular atria in 100 healthy fetuses with gestational ages ranging from 14 to 38 menstrual weeks were evaluated and compared with those of 38 fetuses in whom ventriculomegaly had been diagnosed in utero. Axial sonograms of the brain through the atrium of the lateral ventricle demonstrated that the normal atrial diameter remained relatively constant throughout the gestational age range observed. The atrium had a mean diameter of 7.6 +/- 0.6 mm (standard deviation [SD]). Measurement of this structure can be quickly performed, is reproducible with low intra- and interobserver variation, and permits ventriculomegaly to be excluded. Atrial diameters exceeding 10 mm (above 4 SDs) suggest ventriculomegaly, with a low false-positive rate.