Metal-on-metal prostheses are increasingly and widely used in total hip arthroplasty, and offer particular benefit to patients with osteoarthritis. Adverse effects related to the release of metal ions (such as cytotoxicity, genotoxicity, carcinogenicity and metal allergy) are common, however. The aims of this review article were to explore the relationship between corrosion products and implant-related hypersensitivity, define normal and toxic metal ion concentrations, and differentiate between allergy and infection in painful total hip replacement. The simultaneous presence of corrosion products and hypersensitivity-related tissue reactions indicates a relationship between the development of corrosion and implant-related hypersensitivity. There are no clear boundaries between normal and toxic metal ion concentrations. Several methods exist for the differential diagnosis of metal allergy and infection, including ultrasound-guided aspiration, patch testing and arthroscopic biopsy. More research is required to elucidate fully the relationship between metal articulations and allergy, and to determine the concentrations of metal ions that lead to harmful effects.