Intracutaneous testing and patch tests with house dust mite and grass pollen allergens were performed in patients with atopic dermatitis. Only patients with an immediate type skin reaction to house dust mite or grass pollen allergens showed a positive patch test reaction to these allergens 24-48 h after testing. Occasionally positive patch test reactions at 20 min, 2 h and 6 h were also observed. Patch test reactions were not found in normal controls or atopic patients without atopic dermatitis. Analysis of the cellular infiltrate demonstrated an influx of eosinophils into the dermis, starting from 2-6 h after patch testing. Immunostaining with antibodies against granular constituents of the eosinophils revealed that the infiltrating eosinophils were in an activated state and had lost part of their granular contents. At 24 h eosinophils also appeared in the epidermis. Electron microscopy showed that in the epidermis, some eosinophils were in close contact with Langerhans cells, suggesting a cell-cell interaction. Taken together, these results strongly suggest an active role for eosinophils in patch test reactions to inhalant allergens in atopic dermatitis patients.