Background. The long-term impact of dividing Piriformis when performing a standard posterior approach to the hip has not been assessed. A less invasive approach in which the tendon is preserved has been described (the Piriformis-sparing minimally invasive [PSMI] approach). With advances in MRI technology it is now possible to image structures in close proximity to metal without artifact. The aim of the study was to compare the MRI features (muscle grade and bulk) of the divided and repaired Piriformis tendon group (by the standard posterior approach) with the preserved tendon group (by the PSMI approach). Methods. As part of an on-going randomized controlled trial comparing the PSMI approach with the standard posterior approach for total hip replacement, a subgroup of 20 consecutive patients who fulfilled the inclusion criteria were recruited. Patients received preoperative and 3 months and 2 year postoperative MRI scans. Patients and evaluators were blinded to allocation. Outcomes assessed included grade, volume and integrity of Piriformis. Results. There was significantly less deterioration in muscle grade (p = 0.002) and bulk (p = 0.013) in the preserved Piriformis tendon group in comparison to the divided and repaired Piriformis group. There was equal deterioration in muscle grade and bulk of all divided tendons (Obturator Internus and Piriformis by the standard approach and Obturator Internus by the PSMI approach). The majority (10/11) of the Piriformis tendons by the PSMI approach were intact post-operatively. Conclusions. The PSMI approach, in which the Piriformis tendon is preserved, is successful in maintaining the integrity of the tendon as well as preventing deterioration in grade and bulk of the muscle belly