The subjects in this study cover seven selected patients-four with Chiari II malformation associated with a vanishing cerebellum and three with primary cerebellar agenesis-and the main radiological (MR imaging) differences between the two are discussed. The reason for this discussion is that some authors consider primary cerebellar agenesis as a type of Chiari malformations, and refer to it as Chiari IV malformation. Primary cerebellar agenesis can be distinguished from the Chiari II malformation associated with a vanishing cerebellum by presence of a relatively normal-sized posterior fossa, identification of symmetrical remnants of the anterior quadrangular lobules, by lack of scattered remnants of cerebellar tissue, and by presence of a normal brain and spine. The only common finding in both conditions is a small brainstem with loss of normal pontine configuration. Considering the previous descriptions of anatomical changes related to the so-called Chiari IV malformation which include absent cerebellum, and large posterior fossa cerebrospinal fluid spaces, it is probable that many of the cases previously considered as Chiari IV malformation were unrelated to any type of Chiari malformations, and actually represented primary cerebellar agenesis.