To determine whether morphological differences in the response of cutaneous mast cells characterize clinically distinct forms of urticaria, we used ultrastructural techniques to examine skin biopsy specimens from three patients with cold-induced urticaria and four patients with dermographism. Biopsy specimens were obtained before application of the stimulus and at the time of lesion formation. Patients with cold-induced urticaria exhibited morphological alterations only after stimulus application consisting of enlargement and uniform disorganization of some, but not all, granules, fusion of the membranes of adjacent granules, fusion of granule membranes with mast cell membranes, and discharge of electron-lucent and disorganized granule contents into the extracellular space. Mast cells from patients with immediate as well as delayed dermographism exhibited alterations before and after stimulus application consisting of enlargement of most granules, nonuniform (zonal) disorganization or solubilization of granule contents, fusion of granule membranes with mast cell membranes, and extracellular discharge of granule contents. Small cytoplasmic vesicles containing disorganized granular material were associated with the degranulation process. Endothelial cells lining nearby postcapillary venules exhibited prominent perinuclear condensation of contractile microfilaments during degranulation in both groups. Both before and after application of the stimulus, the walls of the superficial dermal vessels of the patients with dermographism were thinner and contained less extracellular matrix material than vessel walls of the patients with cold-induced urticaria. The morphologically distinctive types of mast cell degranulation that characterize these two clinically separable urticarial disorders may indicate different pathogenic mechanisms of lesion formation.