25 cases of atypical sweat duct proliferation in association with cutaneous tumors are described. 20 of these were keratoacanthomas. The compression of the duct either in its dermal or intraepidermal portion is considered to be the etiology. Similar changes are seen with decubitus ulcer and stasis dermatitis. Atypical sweat duct hyperplasia may be useful as an extra aid in the differentiation of keratoacanthoma from well-differentiated squamous cell carcinoma.