We present transvaginal ultrasonographic findings of a fetus with Dandy-Walker malformation and associated massive obstructive hydrocephalus at 13 weeks' gestation. First-trimester ultrasonographic diagnosis of Dandy-Walker malformation is uncommon with only two such occurrences having been reported previously. These cases and recent reports of single gene transmission of this condition in some families emphasize the importance of first-trimester transvaginal ultrasound assessment especially in women with previously affected fetuses.