Abstract : SUMMARY :Sacro-lumbar fusion must be carefully planned to avoid alterations of the adjacent motion segments. This is a retrospective study to evaluate the fate of transitional segments at mid and long term. The aim was to discuss the value of systematic preoperative MRI before spine fusion. The study group comprises 68 patients who sustained a lumbar or sacro-lumbar fusion for degenerative discopathy or spondylolysis with spondylolisthesis between January 2000 and December 2005. The preoperative evaluation included standard radiographs and MRI. Postoperative follow-up averaged 51 months and ranged from 6 to 121 months. 72% of the patients had no clinical or radiological complications, 18% developed mild lesions such as a moderate disk space narrowing or osteophyte formation and only 10% developed a significant adjacent motion segment alteration. The risk of degeneration increased with age, female sex, length of fusion and when the indication for fusion was a degenerative pathology. Compared with other studies, this work reports few true adjacent motion segment alterations because preoperative MRI allowed a better selection of the future adjacent segment.