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.