The spines of 28 cadavers were studied with magnetic resonance (MR) imaging, correlated with anatomic sections, to characterize the progressive and regressive changes in the nucleus pulposus. Four additional types of nucleus pulposus were identified in this study. The transitional type (type II) of childhood had fibrous tissue that developed near the anulus fibrosus anteriorly or posteriorly. MR images showed a notch of reduced signal intensity in the nucleus pulposus where the fiber was developing. The adult type (type III), which is predominant after age 30 years, had an indistinct boundary between the nucleus pulposus and anulus fibrosus. MR images showed the fibrocartilage in the nucleus pulposus and inner anulus fibrosus as areas of high signal intensity. Adult disks with a tear of the anulus fibrosus (type IIIB) had a lower signal intensity on MR images and slightly reduced disk heights. This type may represent an early stage of degeneration. Degenerated intervertebral disks (type IV) had either dense disorganized fibrous tissue or fluid replacing the normal fibrocartilaginous structure of the nucleus pulposus. MR images showed narrow intervertebral disk spaces with either reduced signal intensity if collagen was present or increased signal intensity if fluid was present. This study suggests that the nucleus pulposus may progress normally through types I, II, and III.