Nucleus pulposus replacement and augmentation has been proposed to restore disk mechanics in early stages of degeneration with the option of providing a minimally invasive procedure for pain relief to patients with an earlier stage of degeneration. The goal of this paper is to examine compressive stability of the intervertebral disk after either partial nucleus replacement or nuclear augmentation in the absence of denucleation. Thirteen human cadaver lumbar anterior column units were used to study the effects of denucleation and augmentation on the compressive mechanical behavior of the human intervertebral disk. Testing was performed in axial compression after incremental steps of partial denucleation and subsequent implantation of a synthetic hydrogel nucleus replacement. In a separate set of experiments, the disks were not denucleated but augmented with the same synthetic hydrogel nucleus replacement. Neutral zone, range of motion, and stiffness were measured. The results showed that compressive stabilization of the disk can be re-established with nucleus replacement even for partial denucleation. Augmentation of the disk resulted in an increase in disk height and intradiskal pressure that were linearly related to the volume of polymer implanted. Intervertebral disk instability, evidenced by increased neutral zone and ranges of motion, associated with degeneration can be restored by volume filling of the nucleus pulposus using the hydrogel device presented here.