The immunopathology of patch test reactions to aeroallergens in patients with atopic dermatitis (AD) has been compared to that of contact sensitivity reactions to nickel in non-atopic individuals. Both reactions were found to exhibit equivalent erythema and induration on gross examination at 48 h. Four millimetre punch biopsies were obtained at 48 h frozen, and cryostat sections stained with a panel of MoAbs. The distribution of macrophages, dermal dendritic cells, Langerhans cells, T lymphocytes and the expression of CD23 antigen was recorded. Increased numbers of dermal dendritic cells, macrophages, T lymphocytes and Langerhans cells were found in the dermal infiltrates of both the nickel patch test reactions and the aeroallergen patch test reactions compared with their respective controls. There were no significant differences between atopic patch test reaction and nickel patch test reaction samples in the tissue distribution of these cell types. There was a significant increase in CD23 expression on Langerhans cells and dermal dendritic cells in the atopic patch test reactions, whereas an increase was only observed on dendritic cells in nickel patch test reactions. No significant difference in CD23 expression was observed in the control skin samples taken from patients with AD, nickel-sensitive patients and normal controls. This study supports the hypothesis that the aeroallergen patch test reaction in atopic dermatitis is a delayed hypersensitivity reaction, yet is distinct from the contact sensitivity reaction to nickel in terms of raised expression of CD23.