A case report is given on a woman with rheumatoid arthritis and deformed knee joints. She got a knee arthroplasty with a metal-to-metal Vitallium endoprosthesis ad modum Walldius. About one year after the arthroplasty she showed both a sterile fistulation from the knee joint and an eczema of the skin of the operated knee. An epicutaneous test for cobalt was positive. It is suggested that the metal-to-metal contact in the prosthesis and the proximity of the prosthesis to the skin is a possible explanation to the allergic reaction.