BACKGROUND: We hypothesized that by using coronal MRI, Chiari I malformation could be more precisely diagnosed, would provide simple anatomic landmarks, would provide information regarding asymmetry of hindbrain herniation, and would be a better method for analyzing the tonsillar herniation postoperatively when the opisthion has been removed. METHODS: Fifty consecutive pediatric patients diagnosed with Chiari I malformation had comparison between the measurements of their caudally descended cerebellar tonsils on midsagittal and coronal MRI images. RESULTS: On MRI coronal imaging, tonsillar asymmetry was found in 48 patients. Maximal left tonsillar descent was 20.9 mm, and maximal right tonsillar descent was 17.4 mm. On MRI sagittal imaging, tonsillar descent ranged from 5 to 27.4 mm. Fifty-eight % of patients had syringomyelia. Five patients (10 %) on coronal MRI were found to have both cerebellar tonsils that were less than 3 mm below the foramen magnum. However, all of these patients had greater than 3 mm of tonsillar ectopia on sagittal imaging. Nineteen patients (38 %) on coronal MRI were found to have one of the cerebellar tonsils that were less than 3 mm below the foramen magnum. Similarly, each of these had greater than 3 mm of tonsillar ecotpia as measured on midsagittal MRI. Also, based on these findings, Chiari I malformation is almost always an asymmetrical tonsillar ectopia. CONCLUSIONS: Sagittal MRI overestimates the degree of tonsillar ectopia in patients with Chiari I malformation. Misdiagnosis may occur if sagittal imaging alone is used. The cerebellar tonsils are paramedian structures, and this should be kept in mind when interpreting midline sagittal MRI.