Metallosis represents a rare and severe complication of knee replacement surgery. It is caused by the infiltration and accumulation of metallic debris into the peri-prosthetic structures, deriving from friction between metallic prosthetic components. In knee arthroplasty, this event generally occurs as a result of polyethylene wear of the tibial or metal-back patellar component. The real incidence of metallosis is still unknown, although it seems to be more frequent in hip than in knee arthroplasty. The metallic debris induces a massive release of cytokines from inflammatory cells, making a revision necessary whenever osteolysis and loosening of the prosthesis occur. We report four patients who underwent revision of their knee arthroplasty because of severe metallosis. In one of these patients, polyethylene wear had determined friction between the metal-back patellar component and the anterior portion of the femoral component. In the remaining three cases, metallosis was caused by friction between the femoral and tibial prosthetic metal surfaces, resulting from full-thickness wear of the ti~ial polyethylene. T lymphocytes were activated by metal particles present in periprosthetic membranes. In all patients, one-stage revision was necessary, with rapid pain disappearance and a complete functional recovery of the knee joint. Metallosis represents a rare and dangerous complication of total knee arthroplasty (TKA). It is characterized by the infiltration and accumulation of metal debris into the peri-prosthetic structures, including bone and soft tissues (1-3). The debris derives from friction among metal prosthetic surfaces, mainly due to the wear of the tibial and/or the metal-backed patellar polyethylene component. Metallosis seems to be more frequent in hip replacement, being the result of the metal-on-metal coupling (4). The exact incidence of metallosis following TKA is unknown. Chang et al. reported an overall 5.3% incidence of metallosis in 418 total hip arthroplasties (5); Ollivere et al. reported a 3.1% rate of metallosis-related failure at five years in 463 primary metal-on-metal hip resurfacing arthroplasties (6). Metallosis has also been described in shoulder arthroplasty (7-13).The presence of intra-articular metallic debris can be detected by x-ray imaging, this condition being an indication for early revision (1,3,5). Furthermore, metallosis can cause peri-prosthetic osteolysis, as the consequence of the release of cytokines from the inflammatory cells, inducing pain and implant loosening (14-19): this condition makes implant revision essential in order to prevent further bone loss.All orthopaedic implants are heading towards