Gold sodium thiosulfate, inserted in the patch test standard series, gives a surprisingly high yield of positive reactions, about 10% of tested eczema patients, but with a low degree of clinical relevance. A histopathological study on patch test reactions was carried out in patients selected because of a combined contact allergy to gold sodium thiosulfate and nickel. Biopsies were taken from macroscopically similar reactions in dilution series of each allergen. The histological picture was clearly eczematous, without irritative features. In a blind comparison, test reactions induced by the two allergens could not be differentiated from each other. Long-lasting patch test reactions to gold sodium thiosulfate were characterized by an intense lymphocytic dermal infiltrate without epidermal involvement. Immunohistochemically, CD4+ and CD8+ T lymphocytes could be detected already 4-8 h post-challenge, and "naive" as well as "memory"-type T cells were demonstrated. Apparently, our findings reflect a true contact allergy to gold sodium thiosulfate.