We present a case series of metal on polyethylene total hip replacement (MoP THR) with delayed onset dislocation, associated with unrecognised adverse local tissue reaction to trunnion corrosion and pseudotumour formation. The diagnosis was not suspected in nine of the ten cases, despite the subspecialty nature of the unit at which they were treated. Instead, it was identified at the time of revision surgery, because of the intraoperative findings, subsequently confirmed by histological examination of the resected tissue. Preoperative assessment and culture results ruled out infection.A variety of treatment strategies were employed, including tumour resection and efforts to avoid recurrent dislocation. The rate of complications was high and included three deep infections, two cases with recurrent dislocation, and one recurrent pseudotumour. This series demonstrates that pseudotumour is an infrequent but important cause of delayed instability following MoP THR, and it is easy to overlook in the differential diagnosis, especially if the original implant alignment is less than optimal, leading to an assumption of component malalignment as the cause of the dislocation.Revision is complex, requiring resection of the pseudotumour and modification of the reconstruction. The risk of postoperative complication is high.