High-field-strength MR imaging was performed in one patient with Chiari III and 19 patients with Chiari II malformations. The MR features were compared with descriptions in the literature and correlated with previously described surgical and postmortem findings and with the results of previous radiologic investigations in this group of patients. Several new observations were apparent from the MR examinations. In 75% of the 20 cases, the underdeveloped tentorium with a wide incisura allowed inferior displacement of the medial posterior cerebrum, which appeared closely applied to a flattened aspect of the superior cerebellum. Previously reported CT descriptions of "pseudotumor of the tentorium" and "towering cerebellum" may be more related to the technique of the radiologic examination than to true upward herniation of the cerebellum. Elongation of the mesencephalon with increase in the mamillopontine distance was present in the majority of our cases and has not been previously emphasized. Some patients had atypical changes or appeared to be borderline cases between the Chiari I and Chiari II categories of malformation, and MR provided considerable diagnostic assistance in these cases. The noninvasive, in vivo evaluation of MR contributed a great deal to our further understanding of this congenital Chiari malformation.