STUDY DESIGN: We retrospectively evaluated the value of magnetic resonance imaging (MRI) in the diagnosis of lumbar lateral disc herniations. SUMMARY OF LITERATURE REVIEW: MRI is known to be a reliable study for the diagnosis of a lumbar disc herniation. However, recent studies of its diagnostic value for lateral disc herniation have been rare. OBJECTIVES: We aimed to assess the diagnostic value of simple MRI, to determine the need for additional imaging studies and to investigate mimicking lesions. MATERIALS AND METHODS: In a lateral herniation group composed of 21 cases, including 10 foraminal and 11 extraforaminal herniations, the diagnostic value of simple MRI was evaluated, and the potential requirement for additional studies investigated. In a mimicking lesion group(5 cases), the entity of each lesion was identified. RESULTS: All 10 foraminal disc herniation cases were able to be confirmed with simple MRI, six of which were confirmed using sagittal images alone. In contrast, for the eleven extraforaminal disc herniations, sagittal MR images were not at all helpful in the diagnosis; however, six(55%) were confirmed from axial images, but the other five could not be confirmed until additional studies, such as enhanced MRI(4 cases), 1 mm-sliced CT (1) and CT-discography (3), were carried out. All 5 mimicking lesions were upper endplates of the lower vertebrae. CONCLUSIONS: Simple MRI is useful in the diagnosis of foraminal herniations, but not so helpful for extraforaminal herniations; particularly, sagittal images are of little use. Therefore, whenever a patient complaining of severe radiating pain presents with no causative finding on simple MRI, the extraforaminal regions on the axial images should be diligently scrutinized again, and additional studies considered when necessary. Conversely, mimicking lesions, such as an upper endplate, should be differentiated when a lateral disc herniation is suspected.