Abstract: Ultrastructural changes of chronic urticarial wheal in 11 patients are reported. It could be seen under the TEM that the capillaries of superficial dermis were dilated and had a diameter of 20μ, but some of them became narrower and with a diameter of 2μ. The endothelial cells of capillaries became edematous and their processes became flat. Many vacuoles could be found in the cytoplasm and the intercellular space widened. Scattered infiltrates of lymphocytes, plasma cells, mast cells and eosinophils could be found around the blood vessels. Degranulated mast cells could also be seen. The edematous dermis was filled with some amorphous material of low electron density. Swellen collagenous fibres could be found in the edematous papil-lary layer. The keratinocytes in the epidermis were edematous too. The causes of edema in chronic urticaria were discussed.