Sixteen cats with allergic dermatitis and six control cats with no skin disease were examined. Lymphoid and histiocytic cells in skin sections were examined immunohistochemically and mast cells were identified by toluidine blue staining. The 16 allergic cats showed one or more of several features (alopecia, eosinophilic plaques or granulomas, papulocrusting lesions), and histopathological findings were diverse. In control cats there were no cells that expressed IgM or MAC387, a few that were immunolabelled for IgG, IgA or CD3, and moderate numbers of mast cells. In allergic cats, positively labelled inflammatory cells were generally more numerous in lesional than in non-lesional skin sections, and were particularly associated with the superficial dermis and perifollicular areas. There were low numbers of plasma cells expressing cytoplasmic immunoglobulin; moderate numbers of MHC II-, MAC387- and CD3-positive cells; and moderate to numerous mast cells. MHC class II expression was associated with inflammatory cells morphologically consistent with dermal dendritic cells and macrophages, and epidermal Langerhans cells. Dendritic cells expressing MHC class II were usually associated with an infiltrate of CD3 lymphocytes, suggesting that these cells participate in maintenance of the local immune response by presenting antigen to T lymphocytes. These findings confirm that feline allergic skin disease is characterized by infiltration of activated antigen-presenting cells and T lymphocytes in addition to increased numbers of dermal mast cells. This pattern mimics the dermal inflammation that occurs in the chronic phase of both canine and human atopic dermatitis.