With the increasing use of pedicular system to fix the spine, many complications are being reported. Recently many systems are available to fix the spine. However, each system has own advantages and disadvantages. The causes of metallic failure of Modular Segmental Spinal (MOSS) instrumentation on 42 consecutive patients undergone in Kang-Nam St. Mary's hospital since 1989 were reviewed. The specific aim of this investigation was to assess causes of metallic failure and loosening of this system on various spinal disorders. Of these, 8 cases had metallic failure and loosening. Breakage of screw and rod developed in 6 cases and dislodgement of rod from screw in 2 cases. In case of degenerative spondylolisthesis (unstable phase) with stenosis, however, the complications were closely correlated to expansile decompressive laminectomy to widen the narrowed spinal canal and the instrumental distraction to gain normal intervertebral disc space at the operation. Bony union and back pain were not correlated to metallic failure and loosening. Therefore, the main causes of metallic failure and loosening were (1) preoperative instability undergone expansile decompressive laminectomy including total bilateral facetectomy that aggravated preoperative instability, and (2) forceful instrumental dis traction. In cases needed these requirements, combined anterior interbody fusion or posterior interbody fusion should be added, heavier rods and screws larger than 3.5mm, 4.0mm in diameter, respectively, should be used. In addition to postoperatively sufficient bed rest and immobilization using rigid braces should be recommended to reduce these complications.