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Rationale Hypersensitivity reactions to metal implants have been reported mostly under the picture of local or systemic eczema or impaired wound and fracture healing. In addition, in arthroplasty non infectious complications like swelling, pain or loosening cannot be always explained by mechanical factors. We report on a patient with local seroma and loosening of his hip endoprosthesis, in whom patch testing was negative but periimplantar tissue showed cobalt-specific DTH. Methods Patch testing was done with a standard and an implant constituent series. Immunohistochemistry, RT-PCR for cytokine expression and lymphocyte transformation tests (LTT) were performed. Results Patch test gave no reactions. Immunohistochemistry of periimplantar tissue obtained at revision surgery showed dense lymphocytic infiltration with marked predominance of CD45RO+ memory T-cells. RT-PCR revealed strong tissue expression of TNF-a and IFN-g, but absence of IL-4 and IL-10. Reactivity of peripheral blood mononuclear cells to nickel, chromium, cobalt, molybdenum was not enhanced. But cells derived and expanded (by IL-2) from periimplantar tissue showed marked proliferative response to cobalt (stimulation index 5.2 and 6.2, CoCl3 10–5 and 10–6 M). Conclusions Metal specific hypersensitivity reactions may arise in the proximity of metal implants and not necessarily coinvolve the skin. Due to missing patch test reactivity, incidence of such conditions may be underestimated.