Thirty-three fetuses with an enlarged cisterna magna (10 mm or more) were evaluated to determine factors that might be associated with an underlying chromosome abnormality. Eighteen fetuses (55%) proved to have a chromosome abnormality, including trisomy 18 or trisomy 18 variant (12), trisomy 13 (three), Turner syndrome (one), or other rearrangements (two). Among various risk factors analyzed, the absence of ventricular dilatation correlated most strongly with a chromosome abnormality. Chromosome abnormalities were found in 17 of 22 fetuses (77%) lacking ventricular dilatation, compared with only one of 11 fetuses (9%) with ventricular dilatation (P less than .001). Other factors statistically associated (P less than .01) with an underlying chromosome abnormality included mild enlargement of the cisterna magna (10-14 mm), concurrent anomalies detected sonographically, and fetal growth retardation. However, stepwise logistic regression showed that only the absence of ventricular dilatation and the presence of concurrent anomalies were significant when multiple factors were evaluated. These observations support the utility of evaluating the cisterna magna as part of a routine anatomical survey.