This is a preliminary report of the authors' first 40 patients treated by posterior spinal fusion with Wisconsin interspinous segmental instrumentation (WISSI) with a minimum 18-month follow-up period. No major complications because of the instrumentation were encountered. The patients were allowed to sit in a chair on the second or third postoperative day. Early elimination of postoperative casts resulted in an increased rate of problems with fixation and led to reintroduction of a postoperative orthosis, but not the body cast. Some fixation problems may have been related to technical errors and use of the earlier one-button technique. In two patients who had hook dislodgement, the WISSI maintained spinal correction thereby demonstrating its stabilizing effect.