Abstract:A 4‐month‐old girl had an isolated, reddish brown nodule on her back that blistered repeatedly. Toluidine blue staining of a biopsy specimen showed a dense infiltration of mast ceils in the upper dermis. In addition, large granules that stained similarly to the normal mast cell granules were observed. Electron microscopic studies disclosed that these giant granules (2–4 pm diameter) had the characteristic substructures of mast cell granules, that is, lamellar and scroll‐like forms. Some giant mast cell granules lost their electron density, suggesting a degran‐utatton process. Aggregates of normal‐size granules with varying degrees of electron density were also observed. We assume that trie variations in morphology of these giant granules represent their maturat