We studied four patients (a mother, her two daughters, and her son) with bullous mastocytosis, or diffuse cutaneous mastocytosis, whose genetic inheritance suggested an autosomal dominant pattern. The clinical characteristics included extensive bullae, numerous urticaria, pruritus, flushing, and pseudolichenified skin over all body surfaces without systemic organ involvement. The histopathologic findings disclosed a pronounced accumulation of mast cells in the dermis. Electron microscopic studies of lesional skin obtained in infancy showed round or spindle-shaped mast cells with numerous fingerlike villous protrusions. The cytoplasmic granules varied in size and shape, and the appearance of degranulation was markedly noted. In the adult, most mast cells had markedly decreased numbers of granules and cytoplasmic villi. Some cells displayed degenerative or necrotic appearances. These findings correlated well with the clinical course of these cases, which improved spontaneously over time.