Aims: MRI changes to the symptomatic intervertebral disc following Intradiscal Electrothermal Therapy (IDET), in particular those relating to the Ç High Intensity Zone È (HIZ) in the posterior annulus, were determined in this prospective study. Methods: MRI images before the IDET procedure were compared to those taken at six months post procedure in 10 patients. The presence and absence of an HIZ, the disc height and hydration, and Modic changes, were determined from the images. Two of the patients also had discography performed post-IDET to supplement the MRI. Results: In 6 out of the 10 patients, an HIZ was present on the MRI images of the disc before the IDET procedure. In all 6 patients, a HIZ was still present six months after the procedure. In all 10 patients, there were no changes to disc height and hydration signal on T2 weighted images. Modic changes were not present in any of the patients on pre or post IDET images. Two patients had signiþ-cant changes to the shape of the posterior annulus compared to the pre-treatment MRI scans. The two patients who had discography after IDET had persistent annular tears. Conclusion: Our þndings question the clinical relevance of the Ç High Intensity Zone È. They also suggest that the main mechanism of action of IDET, may be other than that of sealing the annular tear.