We present 2 patients with swelling of the groin after metal-on-metal total hip arthroplasty without radiological signs of component loosening. Magnetic resonance imaging in both patients showed a round intrapelvic lesion ventral to the femoral head. During surgery, we found cystic structures filled with fluid and necrotic masses. After resection of the cyst, the metal head and insert were replaced by a new ceramic head and a polyethylene liner. Although 2 different kinds of alloy were used (Sikomet, low carbon content; Metasul, high carbon content), histopathological analysis in both cases showed typical morphological signs of a hypersensitively determined inflammation. Despite the distinct soft tissue reaction, bony component integration was unaffected. In our opinion, open resection of the cystic lesion and changing of the metal-on-metal articulation is the treatment of choice.