The recommended patch test procedure for nickel allergy with 5% NiSO4 in petrolatum was evaluated in a total of 169 female twins selected on the basis of a positive history of metal sensitivity. At the time of the evaluation most of them were healthy without current dermatitis. Of 88 female twins with a history of nickel sensitivity 62 (72%) had positive reactions after patch tests with 5% NiSO4 in petrolatum. In the remaining 26 the loss of epicutaneous reactivity was independent of age and time lapsed since the last episode of clinical dermatitis, and it did not occur more often in monozygotic than in dizygotic pairs. The number of negative reactions was greater after an exposure time of only 24 h than after 48 h. In 103 consecutive patients 2.5% NiSO4 in 50% DMSO or 5% NiCl in petrolatum were evaluated as possible alternative test substances. These tests produced a large number of irritant reactions and are therefore not suitable for routine use. One patient with a positive history of nickel allergy reacted negatively with 5% NiSO4 in petrolatum but had a true positive reaction with 5% NiCl. Obviously, 5% NiSO4 may give false negative reactions, and alternative test substances can therefore be useful in selected cases. In the present twin material positive nickel patch tests did not produce "angry back" reactions.