To determine whether cerebellar dimensions measured with cranial ultrasonography help assess gestational age in neonates, the authors measured the area, circumference, and vertical length of the vermis and the area, circumference, and maximum transverse width of the cerebellar body in 80 healthy neonates. These dimensions were correlated with the clinically assessed gestational age. All three measurements of the vermis and cerebellar body strongly correlated with gestational age. The area and circumference of the vermis were superior to the maximum transverse cerebellar width in prediction of gestational age. Multiple regression models made with a single vermis measurement along with either the cerebellar area or its circumference further improved gestational age prediction (r = .88; P less than .0001). The cerebellar dimensions also correlated strongly with clinically assessed gestational age in 13 neonates between 500 and 750 g birth weight, 18 neonates small for gestational age, and 12 neonates in whom clinical and obstetric assessments of gestational age were discrepant by more than 2 weeks.