Local cytokine profiles in skin biopsies from allergic and irritant patch test reactions were determined by in vivo immunohistochemistry to differentiate between these 2 clinically identical afflictions especially at the same time of final reading in diagnostic patch testing. Biopsies were taken from established allergic persons after specific allergic patch tests to epoxy resin (1%), and formaldehyde (1%) and from non-allergic individuals with irritant patch tests to sodium lauryl sulfate (10%) and formaldehyde (8%). At 72 h after application of the agents, significantly enhanced frequencies of dermal infiltrating cells, producing IL-alpha, IL-2, and IFN-gamma per 100 infiltrating cells in the dermis, were observed in allergic as well as irritant test patch reactions, as compared to normal skin. Significantly higher frequencies of IL-1alpha producing cells were observed in biopsies from epoxy resin (1%) allergen-affected and formaldehyde (8%) irritant affected skin. The allergic and irritant patch test reactions showed similar levels of expression of the Th1 cytokines IL-2 and IFN-gamma in the dermis, confirmed by probe based detection of IL-2 mRNA and IFN-gamma mRNA. In conclusion, the described similarity shows that allergens and irritants can induce the same profile of IL-1alpha, TNF-alpha, IL-2, and IFN-gamma production, resulting in the near impossibility of discriminating between allergic and irritant contact dermatitis at the time of patch test reading.