Abstract Background The severity and location of adverse local tissue reaction seen in metal-on-polyethylene (MoP) total hip arthroplasty (THA) is not well described. Methods We retrospectively reviewed the records of patients who underwent a revision THA using our biomechanics database. We included all patients who underwent revision surgery for the diagnosis of ALTR with THA implants that had modularity solely at the head-neck junction and excluded patients with implant modularity at sites other than the head-neck junction. Magnetic resonance imaging (MRI) was evaluated by a fellowship trained radiologist who specializes in evaluating metal artifact reducing MRI sequences in order to quantify the ALTR lesions. Histology was evaluated for findings of ALTR using the Campbell score. Results We identified eleven patients in the database. Eight patients had a MRI ALTR grade of severe and seven did based on the histology score. The mean synovial volume was 218,658 mm3 (range 23,461-451,435 mm3) with a mean maximal synovial thickness of 15.3 mm (range 3-34.3 mm). A disruptive infiltration of the abductors due to pseudocapsule invasion was seen in 67% of the patients with three not having abductor involvement. Mean preoperative cobalt and chromium levels were 5.4 ppb (range 1-12.3) and 1.1 ppb (range 0.6-2.4) respectively. Conclusion Patients with ALTR from head-neck junction corrosion in MoP THA may present with large pseudotumors that have previously been under appreciated. Mean preoperative serum cobalt and chromium ion levels remained relatively low, and MRI was an effective way to characterize the size and location of these lesions.