All of the nasal polyps obtained from 83 patients had mast cells which stained with toluidine blue in their tissue. Degranulated mast cells occurred much more in the deep stroma than in the part near the surface of the polyp. More degranulation of mast cells should have occurred in the superficial tissue than in the deep stroma if the inhaled allergen had reacted with specific IgE antibody on the mast cells in the polyp. The degranulation in the deep stroma in the nasal polyp means that another mechanism besides IgE-mediated degranulation must be operative in the mast cells. The pedicle of the nasal polyp had more mast cells and more degranulated ones than had the body and apex. Thus the pedicle is probably more important in the etiology of polyp formation in nasal mucosa than the body and apex from the viewpoint of the activity of histamine from degranulated mast cells on the blood vessels and nasal glands.