INTRODUCTION: Persistence of allergen and immunocompetent cells at sites of healed contact dermatitis has been reported. Flare-up reactions triggered by patch testing and after systemic provocation with allergen are well-known phenomena. To our knowledge, we report the first flare-up of a previous patch test site following casual cutaneous application of nickel in an individual with hitherto latent nickel sensitization. CASE REPORT: Patch testing in a 23-year-old female patient was performed for dermatitis following application of various gels and adhesive bandages: positive delayed-type hypersensitivity reactions were noted for nickel sulfate and potassium dichromate. The patient had never noticed skin reactions to nickel-containing items before. Three weeks following these patch tests, the patient wore earrings which in the past had been well tolerated. She subsequently developed dermatitis of both earlobes within hours and dermatitis at the site of nickel patch testing within a day. CONCLUSIONS: Nickel exposure for 48 h in a patch test is sufficient to induce overt delayed-type hypersensitivity on re-exposure with a previously tolerated antigen in a previously clinically unresponsive individual. Antigen and/or antigen-specific effector cells at the site of previous positive patch testing can be recruited into a delayed-type hypersensitivity reaction for a prolonged period of time.