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Objective:To discuss the relationship between the Chiari- Ⅰ deformity and the perioccipital foramen magnum region deforrnmity in imaging, clinical and pathological manifestation. Methods: Forty one cases of Chiair- Ⅰ deformity in which 28 cases had undergone MRI, 29 cases for CT, and 9 cases for both these two examinations were studied. Results: There was the tonsilla cerebelli herniation in all CT cases which had extent downward from foramen magnum, 25 to C1 bottom level, 4 down to C2 level and below. The tonsilla cerebelli as a soft mass is posterior to cervical spinal cord in magnum foramen level in all axial imaging. In 8 of 9 cases of CTM show the cervical spinal cord deformation due to tonsilla cerebelli compression. Sagittal reconstruction images show that the tonsilla cerebelli extended downward to the spinal canal from the posterior fossa. All 28 cases of MRIgroup showed wedge-shaped tonsilla herniation extending 5 ～ 8 mm down ward magnum foremen; non indicate pathologic characteristics of Chian- Ⅰ deformity. Conclusion: The cervical and rnmedullary curvature was due to the posterior fossa invagination but not to the Chiari- Ⅰ deformity itself.