OBJECTIVE: Metal-on-metal hip resurfacing arthroplasty (MoMHRA) has become a popular option for young patients requiring hip replacement. A recognised complication is the formation of a symptomatic reactive periprosthetic soft tissue mass (pseudotumour). We present a radiological classification system for these reactive masses, dividing them into three groups: Type I are thin-walled cystic masses (cyst wall 3 mm, but less than the diameter of the cystic component) and Type III are predominantly solid masses. MATERIALS AND METHODS: We reviewed all MRI performed over a 4-year period in patients with primary MoMHRA referred to our institution. In all cases the masses were assessed on MRI according to size, anatomical position, signal intensity and involvement of bone, muscle or neighbouring neurovascular bundles. RESULTS: Periprosthetic masses were seen in 33 hips in 17 female (7 bilateral) and 8 male patients (1 bilateral). The Type I lesions were the most common and more likely to be posterior to the hip joint. The Type III masses were significantly larger than the cystic lesions and were more likely to be located anterior to the hip joint. To date 22 patients have undergone revision surgery with conversions to total hip replacement. Severity of symptoms and revision rates were lowest in the Type I group and highest in the Type III group. CONCLUSION: Solid anterior pseudotumours were most likely to have the more severe symptoms and require revision surgery.