Enlargement of the cisterna magna has been reported to be associated with aneuploidy. In prior studies of cisterna magna enlargement, however, those fetuses with abnormal chromosomes have had other sonographic abnormalities in addition to a large cisterna magna. Our goal was to assess the clinical significance of the isolated finding of a cisterna magna measuring more than 10 mm in anteroposterior dimension on a prenatal sonogram. We retrieved all prenatal sonograms performed at our institution between 1989 and 1996 in which an enlarged cisterna magna was the only sonographic abnormality. Cases were included in our study if the cisterna magna measured more than 10 mm in the appropriate plane and the fetal survey was otherwise normal, including normal cerebellar size and morphology. Pregnancy outcome and postnatal follow-up were obtained in each case. Fifteen cases comprised our study population. In all 15 fetuses, the enlarged cisterna magna was first seen in the third trimester (gestational age range, 26 to 37 weeks). The cisterna magna ranged from 11 to 19 mm in size (mean, 12.9 mm). All 15 pregnancies resulted in phenotypically normal liveborn infants. All the mother and infants had short hospital stays (1 to 4 days), and the infants were normal at discharge. Longer follow-up was available in eight cases (range, 2 to 69 months), and all eight of these infants were normal. Our results suggest that isolated enlargement of the cisterna magna to more than 10 mm is associated with normal pregnancy and neonatal outcome.