Differential development of regions of the human cerebellar vermis was evaluated immunohistochemically and morphometrically between 18 weeks of gestation and 10 years of age. The density of Purkinje cells in the cerebellar vermis decreased rapidly until 38 weeks of gestation and slowly thereafter. At all stages of development, the density was higher in the posterior (lobules VI-IX) than the anterior vermis (lobules I-V). The area of cut sections of the anterior and posterior vermis in the mid-sagittal section increased rapidly before 40 weeks of gestation and gradually after birth, whereas growth was slower in the nodules. These developmental characteristics may be related to the selective susceptibility of cerebellar regions to environmental insults.