The role of the eosinophil in the pathogenesis of cutaneous diseases is not known. The eosinophil granule major basic protein (MBP), constituting the core and accounting for greater than 50% of the eosinophil granule, is toxic to helminths and mammalian cells. To determine whether eosinophil degranulation occurs in lesions of chronic urticaria, we performed an indirect immunofluorescence assay on sections of formalin-fixed, paraffin-embedded tissue, utilizing affinity chromatography-purified antibody to MBP. Twelve of 28 biopsies showed evidence of degranulation as judged by the deposition of MBP outside the eosinophil. The positive staining was of 3 types: (1) small blood vessel walls (5 patients), (2) dispersion of granular material (9 patients), and (3) focal or diffuse immunofluorescence of connective tissue fibers (11 patients). These results suggest a possible role for the cytotoxic molecule MBP in the evolution of lesions of chronic urticaria.