Background. Pseudotumours have been associated with metal-on-metal (MoM) hip replacements. We define it as a solid mass which may have cystic components that is neither neoplastic nor infectious in aetiology. The cause of a pseudotumour is not fully understood but could be due to excessive wear, metal hypersensitivity or due to an as-yet unknown cause. Aim. We present the retrieval analysis of early failure MoM hips revised for pain, loosening or a symptomatic mass. Tissues and implants were examined for the possible causes of failure and pseudotumour formation. Corrosion as a potential new cause for pseudotumour formation will be presented. Methods. A group of 16 MoM hip replacements were collected for retrieval analysis. Six of which had a pseudotumour. An Artificial Hip Profiler (Redlux Ltd) was used to measure wear. Edge loading was determined using the 3D wear data. Tissues were histologically evaluated using a 10-point ALVAL scoring system, which strongly suggests hypersensitivity (1). Cases were assigned to one of three categories: high wear (rates >5m/yr), hypersensitivity or corrosion. Results. Of the 6 pseudotumours, 3 had edge loading, 2 had high ALVAL scores and one had corrosion at the head taper junction. The high wear group, (3 cases) all demonstrated edge loading. Histology revealed more metal wear particles and macrophages, with a low ALVAL score in these cases. Two cases were found to have hypersensitivity with a high ALVAL score, more lymphocytes with less visible wear debris and macrophages. The wear rates were also lower, <4m/yr. One total hip replacement had corrosion at the junction between the head and adaptor taper for the stem. The bearings had low wear and the tissue had extensive necrosis as well as products of corrosion enclosed in fibrin (2). There was no suggestion of hypersensitivity (low ALVAL score). Conclusion. Pseudotumours can be caused by high wear, but if this is ruled out, a hypersensitivity or corrosion product reaction should be considered