In rare cases, normal allergy patch tests persist for weeks or months. Here, seven biopsies from six patients with long-lasting allergy patch tests, taken 15-75 days after allergen challenge, were analysed by transmission electron microscopy. In accordance with clinical expression of the test site, fine structural evidence of persistent activity of immunocompetent cells in the epidermis and dermis was observed. Moderate numbers of exocytic lymphocytes and macrophages were seen in the epidermis. Langerhans' cells (LC) appeared active, and LCs with Birbeck granules were detected both in the epidermis and the dermis and were also seen to cross the dermoepidermal junction. LC-mononuclear cell contacts were more frequent in the epidermis than is normal in allergy patch tests. A strong infiltrate of lymphocytes, macrophages, fibroblasts, mast cells, and cells resembling LCs but devoid of Birbeck granules, i.e. indeterminate cells, was seen in the upper dermis. Less than 10% of the dermal cells showing the fine structural appearance of LCs had Birbeck granules, as seen in a single section. Both indeterminate cells and LCs were apposed by lymphocytes in the dermis, often in a rosette-like fashion. The present paper is descriptive but gives background to the understanding of events in long-lasting allergy patch tests. However, the reason for and the basic mechanisms of long-lasting allergy patch tests are not known. A defect in down-regulation of the contact hypersensitivity reaction and/or a constant antigen stimulation could be responsible for the persistence of these allergy patch test reactions.