Although blood eosinophilia is commonly present in atopic dermatitis, accumulation of tissue eosinophils is not prominent. To determine whether eosinophil degranulation occurs in lesions of atopic dermatitis, we analyzed tissues by immunofluorescence for the presence of the eosinophil-granule major basic protein. Twenty biopsy specimens from 18 patients with atopic dermatitis were studied, and all showed major basic protein staining outside eosinophils. In 18 specimens, the staining was fibrillar, was located in the upper half of the dermis, and was similar to the distribution of elastic fibers. Twelve specimens with fibrillar staining also showed major basic protein staining in the form of extracellular granules. One specimen from unaffected skin showed minimal faint, fine, fluorescing fibrils, but there was marked deposition of the protein in affected skin. The fibrillar pattern of major basic protein staining in atopic dermatitis was very similar to that seen in lichenified lesions of untreated onchocerciasis. These results suggest that eosinophils commonly release granule proteins in the dermis and that assessment of eosinophil involvement in disease cannot be based simply on numbers of eosinophils in tissue.