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 time of final reading in diagnostic patch testing. Biopsies were taken from established allergic persons after specific allergic patch test.-, 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-1α, TNF-α. IL-2. and IFN-γ per 100 infiltrating cells in the dermis. were observed in allergic as well us irritant patch test reactions, as compared to normal skin. Significantly higher frequencies of IL- Iα-producing cells were observed in biopsies from epoxy resin (1%) allergen-affected and sodium lauryl sulfate (10%) irritant-affected skin as compared to formaldehyde (1%) allergen-affected skin. In addition, significantly higher frequencies of TNF -α reproducing cells were observed in epoxy resin allergen-affected skin us compared to Formaldehyde (1%) allergen-affected and formaldehyde (8%) irritant affected skin. The allergic and irritant patch test reactions showed similar levels of expression of the Thl cytokines IL-2 and IFN-γ in the dermis. confirmed by probe based detection of IL-2 mRNA and IFN-γ- mRNA, In conclusion, the described similarity shows that allergens and irritants can induce the same profile of IL-la. TNF-α. IL-2. and IFN-γ production, resulting in the near impossibility of discriminating between allergic and irritant contact dermal is at the lime of patch test reading.