The potential of metal-containing orthopaedic prostheses to induce problems through metal allergy taxes dermatologists and orthopaedic surgeons alike. Metal-on-plastic joint replacements are not thought to induce metal allergy but wear products, principally polypropylene particles, produce a foreign body reaction in bone and may lead to aseptic loosening of the joint. Orthopaedic surgeons are increasingly using metal-on-metal joint replacements, particularly for younger patients, as some evidence suggests that there is less wear debris and hence less aseptic loosening. The original metal-on-metal hip joints of the 1960s were associated with sensitivities to cobalt, nickel and chromate when loosening occurred. The potential for modern metal-on-metal joint prostheses, with their lower production of wear debris, to sensitize the recipient to metals or to induce a problem in subjects already allergic to metals, is unclear. One uncontrolled series suggested an association between nickel allergy and prosthesis loosening in some subjects, but the question has yet to be addressed in a prospective study and to date there is no other observation in the orthopaedic literature to suggest a problem.