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A prospective sequential MRI study was done to investigate the morphologic changes of the lumbar disc hernia (LDH). We also studied the relationship between the MRI changes and the type of LDH and the clinical outcome. MRI was performed every 3 months from the onset for a maximum of 24 months in 42 patients with radicular leg pain and symptoms definitely diagnosed as caused by LDH. The size of the herniated mass was determined by the ratio of the anteroposterior diameter of the spinal canal to the maximum diameter of the LDH mass on T2-weighted axial images. The clinical outcome was evaluated as excellent, good, or poor depending on leg pain and physical findings. The JOA (Japanese Orthopaedic Association) score for LDH was also used to assess the outcome. Thirty-seven (88%) of the 42 patients showed >50% reduction of the hernia on MRI 3–12 months after onset, and the morphologic changes of the herniated mass were well correlated with the clinical outcome.