We have studied by electron and light microscopy the inflammatory reaction in lesions at various stages of clinical development from a patient with bullous pemphigoid. The evolution of clinical lesions was associated with a sequence of histopathologic events which began with alterations of mast cells and proceeded to infiltration, first with lymphocytes and later with eosinophils and basophils. Mast cells in the papillary and reticular dermis demonstrated a unique, focal, irregular loss of granule contents. Intact eosinophils demonstrated intracytoplasmic losses of granule contents and karyorrhectic and karyolytic eosinophils had released membrane- bound granules. Partially and completely degranulated basophils were present within a fibrin gel which formed in the dermis. Thus, the sequence of histopathologic events in the pathogenesis of bullous pemphigoid includes mast cell granule alterations and release of granule contents from eosinophils which are undergoing nuclear and cytoplasmic damage.