Ten cases of mammary Paget's disease and 10 cases of extramammary Paget's disease were studied for differences of histologic features in them. Based on the epidermal changes alone, Paget cells in 90% of specimens from lesions of extramammary Paget's disease had abundant mucin that stained well, whereas Paget cells in only 40% of specimens from mammary Paget's disease stained for mucin and faintly at that. In 60% they did not stain at all. The conclusion derived from these observations is that the mucin in Paget cells of extramammary Paget's disease is different from that found in cells of mammary Paget's disease. The Paget cells in these two conditions seem to have different origins, i.e., those of mammary Paget's disease ascend to the epidermis from lactiferous ducts, whereas those of extramammary Paget's disease originate in the epidermis itself.