The distributions of mast cells in decubitus, venous stasis, and arteriosclerotic human skin ulcers were studied. Mast cells were found primarily in the papillary dermis but were also numerous in the reticular dermis of the three types of ulcers. There were mast cells inside the epidermis and many more in granulation tissue. Intraepidermal mast cells were more numerous in venous stasis ulcers than in decubitus or arteriosclerotic ulcers. Many of the mast cells in these areas appeared to be intact. There were also some free and intracellular granules that stained like those in mast cells, in necrotic and granulation tissue. These findings raise questions about the role of mast cells in the epidermis and in necrosis and repair of skin ulcers.