STUDY DESIGN Retrospective study on the incidence of reoperation in patients previously treated by instrumented posterior lumbar interbody fusion. OBJECTIVES To answer the following questions: Reoperation rate after PLIF? And is there any influence of the length of fusion on the reoperation rate? SUMMARY OF BACKGROUND DATA The use of different techniques demonstrates that there is currently no ideal procedure for lumbar fusion. The instrumented posterior lumbar interbody fusion (PLIF) shows a comparable success rate to the so-called 360 degrees fusion techniques (combined dorsoventral spondylodesis) without the need of an anterior approach. METHODS We reviewed 1680 patients who underwent a PLIF at our institution between January 1995 and December 2000. A total of 3053 levels were fused. The reoperation rate was analyzed. The mean follow-up was 5 years. RESULTS There were 221 (13.2%) reoperations in 206 patients (12.2%). Of 1680 PLIFs, 312 were multisegmental (>2 segments). Within this group, 45 (14.4%) revisions were done. We found that the most important difference between the multisegmental PLIFs and the mono- or bisegmental PLIFs is the rate of adjacent segment decompensation (5.1% vs. 2.3%), and this was statistically significant. The reoperation rate between those two groups was only slightly different with 12.9% for mono- or bisegmental and 14.4% for multisegmental PLIFs. CONCLUSIONS The fusion length does not show a significant difference in the reoperation rate as such. Nevertheless, we registered a significantly higher incidence for decompensation of adjacent segments after multisegmental PLIFs.