We studied the natural history of extruded lumbar intervertebral discs using MRI. Forty-nine patients with lumbar disc herniation were included in this study. Ages ranged from 19 to 57. On the T2-weighted sagittal MR image, the signal intensity in the herniated mass was measured and the ratio to that in the original nucleus (i.e., nucleus pulposus from which they extruded) was calculated (signal intensity ratio; SIR). The relationship with SIR and duration of illness was evaluated. In ten patients who were re-examined by MRI after conservative treatment, the size of the herniation measured by T1-weighted axial MR image was compared before and after treatment. The signal intensity of HNP became higher than that of the original nucleus immediately following herniation and thereafter decreased with time, suggesting that initial hydration of the HNP occurred shortly after herniation followed by dehydration of the HNP. The size of the HNP with a SIR value of 1.2 and higher on T2-weighted MR images decrease with time, however, the HNP with a SIR below 1.2 did not show any size reduction. The SIR of 1.2 and higher is a good indicator predicting spontaneous reduction of the HNP. Dehydration in the HNP may play an important role in the reduction of the lumbar disc herniation.