Massive localized osteolysis around artificial joints has been seen more frequently in the past few years. It is still not generally accepted that ultra-high molecular weight polyethylene (UHMWPE) wear particles can induce massive bone resorption, even distant from the joint. This article describes a series of eight soft-top prostheses with large UHMWPE ball heads that contributed to the erosion of surrounding bone. Roentgenographically, all of the cases showed a marked loss of proximal cortical bone, more or less combined with osteolysis, which was distal to the femoral shaft and deep into the acetabulum. In two cases, remodeling and resorption transformed the bone into a tumorlike appearance. Tissue samples from areas of osteolysis as well as from the joint capsule were taken at revision surgery, processed for histology, examined microscopically, and evaluated semiquantitatively. The retrieved devices were also carefully inspected. Large amounts of UHMWPE wear debris were found not only in the joint capsule but also in layers of granulomatous tissue from the acetabulum and femur, whereas metal particles and fragmented polymethylmethacrylate were either completely absent or occurred only in very small amounts. The results of this study demonstrate that UHMWPE wear products alone can cause massive osteolysis by triggering foreign-body granuloma formation at the bone-cement interface; the bone cement may remain fixed in areas beyond the osteolytic transformation.