We report a case of extramammary Paget's disease (EMPD) arising in the knee region in association with an underlying sweat gland carcinoma. Inguinal lymph node metastases were present initially. Paget's cells, intradermal tumor cells, and metastatic tumor cells showed similar reactions to a variety of histochemical stains and positive results with immunohistochemical staining using carcinoembryonic antigen (CEA). Control sections in cases of Bowen's disease and superficial, spreading malignant melanoma showed a negative staining reaction with CEA, providing further evidence that this technique is useful in distinguishing Paget's disease (PD) from histologically similar conditions. We provide a summary of previous cases of EMPD arising in unusual locations and a brief consideration of possible mechanisms of the origin of PD.