Various factors such as hormones, drugs, and ultraviolet (UV) radiation may influence patch test reactions. The aim was to study the individual variation in nickel reactivity, also in relation to the menstrual cycle. Thirty women allergic to nickel were studied for 7 months with patch tests with a serial dilution of nickel sulfate in water on four different test occasions. The patients belonged to two different eczema groups, one with nickel allergy, atopy, and pompholyx (12 patients); and the other with nickel allergy, but without both atopy and hand eczema. None of the patients showed the same patch test reactivity on all four occasions, and the highest individual difference noticed was 250 times for the four test occasions. Furthermore, two of the patients had completely negative test reactions on at least one test occasion. The variation in nickel reactivity as shown in this article is of great importance and should be kept in mind when a patient has a positive history of allergic contact dermatitis but negative patch test results to nickel.